What is a Bug?
A bug is an error in how a program was programmed. Bugs can be logic errors in how an algorithm was written, configuration errors, translation errors,.misspellings, etc. A bugged program typically can't be fixed by its user, only by its developer, however, buggy programs may still be usable with workarounds.
Bugs can have all degrees of severity. Some bugs may be inconsequential but annoying mistakes, like misspellings. Some bugs may be critical, such as those that prevent the program from being used at all, or, in worse cases, corrupt data that is loaded into the program, e.g. if you open a file in the buggy program and save it, you won't be able to open it again because the subprogram that saves the file had a bug in it and it created an invalid file.