this is a nice presentation on known Photoshop tools.

3. MOVE TOOL
The move tool allows you to move objects. You
can select layers with it, so if each of your objects
is on a separate layer, this tool will move those
objects just by clicking on them and dragging. To
copy a selection, press command + option. To
move in a straight line, press shift.

4. MEASURE TOOL
 With this tool we can measure length, width, angle and location of
areas in our image. If we scan something in a little crookedly, we can
use this tool to measure the angle and then use "rotate canvas" to
correct the rotation by the exact amount. To measure, choose the
measure tool from the tool box. Draw a line with it, and look at the info
palette to see the angle, width, length and coordinates of your line.
When you double click on the page, the line will disappear. Then go to
Image > Rotate Canvas > Arbitrary. The amount of rotation needed to
make a 90 degree angle will automatically be computed. Click OK.

5. MAGNETIC LASSO
For shapes that contrast with the background, this
tool will snap to the edge when we make a selection.
Double click on the magnetic lasso icon to open the
dialog box. You can set the strength of the "magnet"
by entering values in the dialog box.

6.  Frequency is the rate at which the lasso will fasten points. Enter a
rate between 0 and 100. Lasso width determines how far the lasso will
reach to find a high contrast edge. Enter a number between 1 and 100.
 Edge contrast tells the lasso how to distinguish the desired edge from
other pixels. A high value detects only images with sharp contrast to the
background. A lower value detects low contrast images. On images with
a well-defined border, you can use a wider lasso width and a high edge
contrast, and you don't need to be precise when tracing. On a border
with fuzzier edges, you should use a smaller width, lower edge contrast,
and trace more carefully.
 If you make a mistake and want to stop drawing, hit escape.

7. FREEHAND PEN TOOL
 This is like the pen tool in any of the other drawing programs. Anchor
points and use handles to make shapes. Click and hold on the pen icon
in the toolbox to see an array of pen tools. The arrowhead on the end is
the vector selection tool. You can use it to select points drawn with the
pen tool. The lines themselves are vector, and won't print out. If we
want to see that shape print, we need to apply a fill or a stroke to the
path.
 To apply a fill, click on the fill path button at the bottom of the paths
palette.

8.  To stroke a path, select the painting tool you want to use,
specify a brush from the brush palette and choose "stroke
path" from the Paths palette menu. You can also click the
stroke path button at the bottom of the paths palette. Each
click of the button makes the path thicker.
 This tool is also great for making a smooth selection. Just
draw as you normally would with the pen tool, and then
choose “Make Selection” from the Paths palette menu or
click the make selection button.

9. MAGNETIC PEN TOOL
Similar to the lasso tool except
that it draws a line instead of a
selection. Great for tracing.

10. TYPE LAYERS
 Adjustment Layers:
 Go to the Layers palette and select "new adjustment layer" or we can also go
to Layer > adjustment layer.
 Adjustment layers work like filters. When we remove them from your image,
the original image remains. This means we can make our tonal adjustments on
layers separate from our image, so they don't become permanent until we
flatten the image.
 To move a selection to a new layer, go to the Layer menu at the top of the
screen and choose "New via Cut" or "Copy" This will either move or copy our
selection to a new layer.

11.  Aligning Layers:
Now layers can be aligned individually to a selection border or to
each other.
 To align a layer to a selection, make the layer active, and
under the layer menu, choose "align to selection". To align two or
more layers to each other, first link them by selecting a layer and
clicking in the box to the left of all layers we want to link to it. Next,
go to the layer menu and choose "align linked". We can use this
method to align text on the page to other text or to selections.
While the text is linked, we can move it as one block.

12.  Layer Effects:
In the Layer palette we will now find a series of effects ,we
can apply to text or other objects on that layer, such as drop
shadow, glow effects, and others.
 Merge Down:
This command in the Layers palette allows we to select a
layer and merge it with the one beneath it. We can change
the order of the layers to merge any two.

13.  Actions :
This function allows us to record a series of actions we intend
to apply to a number of files.
 Creating an action:
1. Go to -- Window > Show Actions.
2. When we are ready to begin recording, then we have to go
to "new action." Type in a name and click record. As we work,
our actions will be recorded.
3. When we are finished, we have to click "stop recording".

14.  To insert a break point, click in the column to the left of the
action. This will pause the action so that we can type in a
value, etc.
 To exclude a command from the list of actions, remove
the check mark at the left of the command by clicking on it.
 To save a series of actions, choose “Save Actions” from
the actions palette menu. We can then choose “Load
Actions” whenever we want to apply those actions to our file.

15. Applying an action to a folder:
 We can apply the recorded actions to a folder of files, and all the files
will automatically go through the recorded actions. To do this, choose
"batch" from the actions palette menu. It will asks us to locate a source.
We can either find one on your hard drive or import files.
 If at any point in our sequence of actions, we need to make a
selection or plug in values, so we can insert a stop or a break point.
 To add a stop, choose "insert stop" from the actions palette. We can
type a message to appear at the stopping point, to remind our self of
what we need to do. If we want to include a continue button, in case no
adjustments are needed, check the "allow continue" button.

16. THE HISTORY PALETTE
The History Palette automatically records and lists
everything we do in Photoshop and allows us to revisit
any previous state in the list. All actions beginning with
the opening file are recorded, and the most recent
action is highlighted. If we want to go back to any earlier
state, we can just click on that state.

17. LINEAR HISTORY
Using linear history, which is the default, all the steps
after crop are greyed out. We can still go back to any of
those states until we begin to alter the image. When we
choose a state and begin to alter the image, our
alterations are saved immediately beneath the state that
we have chosen to work on, and the greyed out states are
lost.

19. HISTORY BRUSH
 Working in the nonlinear history mode allows us to use the
History Brush, with nonlinear history checked, we can choose
any state , select the history brush and click to the left of any
other state. We can see that we have selected the second
Adjustment Layer. Now we can paint with that adjustment layer
on the state just after I cropped but before I made any
adjustments. We can choose brush sizes and shapes by going to
Window > Show Brushes.

21. CHANNEL MIXER
It is often desirable to assign a separate transparent color
to each of two or three images so that when we combine
them, we can easily identify the areas where overlap occurs.
The way to do this is to put each image on its own color
channel. The Channel Mixer automates this process, and
makes aligning the images much easier.

22. HOW TO DO THIS?
1. Open your images. They should be saved as TIFF and the mode
should be set to grayscale. (Go to Image > Mode and choose
grayscale if this is not already the case). If you have only two images to
merge, open a third just as a placeholder.
2. Select one of the images and choose "merge channels" from the
channel palette menu.( Find channel palette under the Windows menu,
and then click on the arrow at the top right of the palette to see
options).
3. Choose RGB color mode.

24. We are then able to assign a color to each of our
grayscale files. Click on the scroll bars until the
correct file is matched with the desired channel. If
we are using a placeholder for our third file, assign
it the extra color. When we have finished, click OK.