When listening to the average person talk you will often hear (and use) words like um, ah, eh, and uh. These aren't necessarily errors or signs of poor communication. These sounds are used to let your communication partners know that you need some time to construct your sentence, and that this is not a real break in the conversation. If you are a good conversationalist, you will avoid interrupting some one who has just finished a sentence with a 'um' sound, indicating that they are still in mid-flow.
Hesitation noises, or filler words*, are useful parts of speech, but should not be overused, as they can become quite annoying.
Seel also: expletive, vocal filler.
*To the best of my knowledge, hesitation noise is the proper term for these. The Cambridge Encyclopedia of Language agrees with me. Filler word is also used, but may not be as formal.