A tense is basically a way of expressing a particular time of an action. It can be in past, which would make it a Past Tense (e.g. Past Simple), in present - Present Tense (e.g. Present Simple) and in the future - Future Tense (e.g. Future Simple).
There are however other ways than just tenses you can express time with, like [am/are + gerund] (I am going/doing/etc.~). We just need to remember that every expression works in the boundaries of some tense, even though it shows different time than the one you would expect, e.g. "I'd rather you didn't go" which uses past form (didn't) to express present/near future.