Vim has several types of registers that store information like deleted or yanked text. The main registers include the unnamed register "" that stores most deleted, changed or yanked text; numbered registers "0-9" that store deletion history; and named registers "a-z" that allow copying or appending text. Other read-only registers store command histories, file names, and the last search pattern. Registers can be listed with :reg and their contents accessed via paste commands to replicate deleted or copied text throughout a Vim editing session.

1. Vim Registers
How to use vim registers, a :h registers tutorial

2. List of Registers
• 1. The unnamed register ""
• 2. 10 numbered registers "0 to "9
• 3. The small delete register "-
• 4. 26 named registers "a to "z or "A to "Z
• 5. four read-only registers ":, ". and "%
• 6. the expression register "=
• 7. The selection and drop registers "*, "+ and "~
• 8. The black hole register "_
• 9. Last search pattern register "/

3. 1. Unnamed register
Any D, C, S, X and Y:
Will fill "" register. Except the _ register.
The "" is used in p, P or :put commands.
Example:
Try yank a line, delete with "_dd and paste it.
The yanked line will be pasted :)

4. 2. Numbered registers
Yank register (0) stores your recent yanked line.
Check your registers:
:reg "0
Paste last yanked line:
"0p
Paste deleted line:
p

5. 3. Small delete register
Read-only register
Referenced by "-
Stores any text that you deleted or changed that
was less than one line in length
Example:
Delete something and check :reg "-

6. 4. Named registers
See full list of your registers:
:reg
Note a quote (") in start of line
How to yank to a register?
<a-z> and <A-Z>!
"ayy OR :yank a
Append to named register:
"Ayy OR :yank A
Cleaning a named register:
qaq
Copying a pattern to register:
:global/Lorem/yank A
See more: :h quote_alpha

7. 5. Read-only registers
":, ". and "%
": Contains the most recent executed cmd line
". Contains the last inserted text
"% Contains the name of current file
!
"# Contains alternate file (can't reproduce)

8. 6. Expression register
In --INSERT-- mode:
<C-r>=(your-expression)
Or in normal mode:
:put =(your-expression)
List of functions: :h function-list
Examples:
2+2
4*4
sqrt(4)

9. 7. Selection registers
"*, "+ and "~
Selects and drops text into the clipboard
Copy to system clipboard:
"+y OR "*y
Paste from system clipboard:
"+p OR "*p OR :set paste, then p
"~ Stores the dropped text from last drag'n'drop

10. 8. Blackhole register
Check your registers:
:reg "
Delete without write in default register:
"_dd
Check again:
:reg "

11. 9. Last search pattern
register "/
Contains the most recent search-pattern
Used for "n" and 'hlsearch'
You can't yank or delete into this register
!

12. Thank you!
@wevtimoteo
!
References:
http://vimcasts.org/results/#stq=registers
http://www.vim.org/docs.php
http://vim.wikia.com