Answer:
A tense is a way of referring to a time (past, present or future) in language. Eg. if you say 'I have walked half a mile today', you are using the past tense because you have finished walking at the moment of speaking.
Within each tense, there is a further subdivision: simple and continuous. If you want to stress the duration of the action of which you speak (the fact that it is/was/will be going on), you use the continuous.
Here are some examples of every tense and its variations:
present simple: I walk
present continuous: I am walking
present perfect simple: I have walked
present perfect continuous: I have been walking
past simple: I walked
past continuous: I was walking
past perfect simple: I had walked
past perfect continuous: I had been walking
future simple: I will walk
future continuous: I will be walking
future perfect: I will have walked
future perfect continuous: I will have been walking