No need for an in depth discussion hear but non the less ratchet extensions are a important piece to a good toolbox. These tools are also called socket extensions or just extension bars. They are used to extend the reach of a ratchet and socket. One end of the extension bar snaps onto the ratchet drive, the other snaps into the socket where the ratchet drive would typically go. Since these act as a ratchet drive extender, you'll need to match the drive size. (1/4", 3/8", or 1/2"). They can usually be purchased in a kit containing all different types of lengths. The more options you have the better, you will be using these tools a lot. You can of course stack multiple extensions together to increase your reach even further.
Extension Bar Options
Here are some feature to look for:
Wobble End: Some extensions have beveled drives that allow a little bit of wobble where the socket snaps on. This allow the socket to be at a slightly different angle than the extension bar.
Locks: Some extension bars come with little locks that will keep the socket from coming off. This is very handy. There nothing more frustrating than trying to retrieve a socket off of a bolt when you can't reach it..
Swivel extensions: these extensions have a u-joint that allows a great amount flex.