What is nickserv?
Nickserv is a bot or "service" found on many IRC networks used to reserve nicknames for users, preventing "nick-theft". This also plays a role in assisting the work of Chanserv and Memoserv (where applicable).

How do I use nickserv?
For a complete listing of nickserv's features, use the command "/nickserv help". (Note that this will work on most networks, if it does not, you may have to use "/msg nickserv" in the place of "/nickserv"), but here are the basics:

Registering a nick
/nickserv register password email
This will register your nickname with the given password, so that others cannot use it. The email address is optional, but is handy so that you can retreive your password if you forget it.
Identifying yourself
/nickserv identify password
This will identify you to nickserv as owning your current nickname. This will grant you access to any chanserv privileges your nick has (eg operator status), and prevent you from having your nick changed automatically by nickserv's enforcement feature.
"Ghosting" yourself
/nickserv ghost password
If your connection dropped for a moment, you may find that a "ghost" of yourself is still on IRC, even though you have reconnected. This command will disconnect that ghost so that you can take your nick back.
Protecting your nick
/nickserv set protect on
This sets it so that if someone else uses your nickname, they will have it forcibly changed unless they provide your password within 60 seconds. The exact syntax of this command can vary, so if it doesn't work check the output of "/nickserv help set" on your network.