Press these 3 keys at the same time: Shift + Command + 3 (Command is the one that has an Apple on it).
An icon will appear on your desktop named Picture 1.png. Click on it to open. Click File
Click Save As
Click on the drop down next to the word Format and choose what you wish to save it as.
I find the Screenshot Plus widget extremely handy, it will allow to select specific windows as opposed to everything on the screen. You can also choose from a number of file formats.