"Either" has a few different meanings: - one or the other of two things, but not both "You can eat either an apple or an orange with your lunch." "Either my mother or my father will drive me to school." "I will either play football or read a book." - a word emphasizing agreement or similarity to a previous statement "You don't like him, do you? I don't, either" "It won't do any harm, but won't really help, either." "I was too tired to go and I couldn't have paid my way, either."

It's used to compare two things. Or telling someone to choosee from two decisions. It really is just "this or that" object, choice, or thing.

I agree with all answers. Please bear in mind that either is coupled with OR. It's negative counterpart is Neither, coupled with NOR. :)

Either/or and neither/nor often come in pairs. For positive statements, I will either add a comment or I will answer the question means that I believe I have two choices and I can pick one. For negative statements, I will neither add a comment nor answer the question says that there are still two choices and I choose to do nothing.

Mike's answer is the best one. It's part of forming a choice, "You can either stay or go". And it's agreeing to a negative statement, "I don't like italki's new design" "I don't either". (I don't mind it really, I just wish it would speed up, and stop throwing errors at me) So it's a choice or concurring to a negative statement.