Dental implants have three basic parts: the fixture embedded in the jawbone, the abutment above the gum line, and the crown. There are different types of implants depending on their placement - endosteal implants are placed within the bone and are most common, subperiosteal are placed under the gum on top of the bone, and transosteal are placed through the bone. Implants can replace a single tooth, multiple teeth, or all teeth by supporting crowns, bridges, or dentures. They improve appearance, comfort, speech, and eating compared to removable dentures, but are more expensive and time-consuming than other options.