Do they ever know who writes them? How seriously do departments take teaching evaluations? If a professor knows which student wrote a particular evaluation....would they be biased (e.g. be nicer, etc...)?

Please include in your answer a short description of what type of school you are at, and what you do there. Answers presumably vary between research universities and four-year colleges, for example. Also, professors and students could reasonably have different access to information about how teaching evaluations are used.