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 email@example.com, e-mails sent to firstname.lastname@example.org 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 email@example.com, firstname.lastname@example.org or U.S.E.R.N.A.M.E@gmail.com will still be delivered to email@example.com.
Additional Gmail e-mail aliases can be created by adding a PLUS (+) and an alias name after your username. E-mails sent to firstname.lastname@example.org, email@example.com or firstname.lastname@example.org will all be sent to email@example.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.