When you choose a Gmail address, you actually get more than just “yourusername@gmail.com.” Here are two different ways you can modify your Gmail address and still get your mail

• Append a plus (“+”) sign and any combination of words or numbers after your email address. For example, if your name was hikingfan@gmail.com, you could send mail to hikingfan+friends@gmail.com or hikingfan+mailinglists@gmail.com.
• Insert one or several dots (“.”) anywhere in your email address. Gmail doesn’t recognize periods as characters in addresses — we just ignore them. For example, you could tell people your address was hikingfan@gmail.com, hiking.fan@gmail.com or hi.kin.g.fan@gmail.com. (We understand that there has been some confusion about this in the past, but to settle it once and for all, you can indeed receive mail at all the variations with dots.)

The real value in being able to manipulate your email address is that it makes it really easy to filter on those variants. For example you could use hikingfan+bank@gmail.com when you sign up for online banking and then set up a filter to automatically star, archive or label emails addressed to hikingfan+bank. You can also use this when you register for a service and think they might share your information. For example, I added “+donation” when I gave money to a political organization once, and now when I see emails from other groups to that address, I know how they got it. Solution: filtered to auto-delete.

Source: Gmail Blog