Crutch words are those words that a speaker use in order to give time for himself to think of the next thing so say. For every language, there probably exists one or more crutch words. Of course, the most popular among English speaker is "uhm." It is no bad to use crutch words but when used excessively, it'll be annoying to the person you are talking to.
Other than uhm, the following words should also be avoided:
Basically - This word denotes a summary in the simplest form. However, it is also now being used to emphasize a point.
Literally - This word should be used to denote a literal or actual situation or condition. However, it is now being used to emphasize a figurative comment.
Like - Probably, like is the crutch word I hate the most because it is being used randomly. Like should denote a comparison of shared likeness or similarity. However, it is now being used in the same category as uhm.