When you are designing a logo for someone, or are having a logo designed by someone, you should always ask that you get it in the following formats: JPEG, PNG, EPS/AI. Each one serves a different purpose, and it's handy to have them all at your fingertips.
Recently I had a client contact me because they were confused which format to use, so I put together this brief post to clear things up.
Here's a breakdown of 3 different file types
and what they're used for: