It is very casual, yes; similar to 'dude' for men. Although 'dude' is sometimes used when talking to a woman, 'dudette' would be more accurate (but this term is seldom used). As far as negative connotations, it may depend on the context. The term 'chick' may be offensive to some, depending on how it is being used and by whom etc.. That said, we typically use the word 'chick' when describing someone (especially strangers). It is very informal and may be considered disrespectful to some.

"Chick" technically means baby chicken. It's slang for young female (16-30). Sometimes it's offensive. Sometimes it's not. That mostly depends on context.

It's a casual slang word for a woman (usually a young woman). It doesn't have any other implications, but it's most commonly used by men when they are talking to other men about a woman.

Casually no, but it depends on the context, it can be too casual to be used to address someone you should speak with more respect, i.e. a journalist talking about the American elections shouldn't refer to Hillary Clinton as a 'chick' or a 'broad', just like they shouldn't refer to a male candidate as a 'dude'.

Offensive, impolite...that's all subjective and relative; sometimes it's justified.

"We were late because some fat chicks were blocking the bread aisle with their carts. They didn't even bother moving for anyone. They just kept talking to each other, while ignoring everyone around them."