A bug is usually a flaw in the logic or design of a program. It is a glitch, a mistake, etc.
Bugs are very, very common.
Way too common if you ask me. In fact, I can't even name one feature-rich program that is bug free. You may wonder how this can be, but remember that programs usually have many more features than a single user ever needs. Other people may demand a feature you don't need. For example, a guy who uses one program at work, may need to be able to load that file type into a different program at home.