When you sign up for a Gmail account from Google, you actually get a couple of e-mail addresses and aliases.
Due to some legal reasons, Google also uses the domain @googlemail.com for their e-mail service besides the usual @gmail.com domain. That means that if your e-mail address is something like username@gmail.com, e-mails sent to username@googlemail.com will also be delivered to your inbox.
Dot or capitalization variations to your Gmail username will still be sent to your inbox. E-mails sent to user.name@gmail.com, u.s.e.r.n.a.m.e@gmail.com or U.S.E.R.N.A.M.E@gmail.com will still be delivered to username@gmail.com.
Additional Gmail e-mail aliases can be created by adding a PLUS (+) and an alias name after your username. E-mails sent to username+school@gmail.com, username+work@gmail.com or username+facebook@gmail.com will all be sent to username@gmail.com.
You can read more about these features here, here and here.
All these e-mail address alternatives can be used to reduce the number of spam e-mails to your Gmail inbox.