Well, an Option type normally is a type that holds either a typed value, or nothing.
For example if you want to get the first integer of a list of integers, that might be an Option<int> because it can be an int or nothing in case the list is empty. But it cannot be a string.
An Either type is just what the name says... either one or the other. And it can be two different types altogether. Either<int, string> would have either an int or a string. Never both. Never none. Either one or the other.
For example a functional ParseInt method might return a Either<int, string>, because it will return either a valid int, or an error message.