In General, the Continuous tenses express actions that are/were "in progress" at a certain time, therefore they are also called "Progressive tenses".

1) Present Continuous or Present Progressive is used, when the action is happening just right now in this moment:
"I'm answering your question right now."
Key words are very often: (right) now, or at the moment.

2) Present Perfect Continuous is used to express an action, that started anytime in the past and is still going on:
"I've been working in the garden for 3 hours". - means that I've worked 3 hours already and I've not yet finished working. I have to go on for another hour.

3) Past Continuous expresses a background action that was in progress, while another event took place:
"It was raining while my dad came home."

4) Past Perfect Continuous is used to express an action that had been in progress for a certain time in the past and stopped before another action started:
"I had been working for 5 hours before I went home"