what is the difference between these two sentences?
I am playing cricket
I am playing cricket now
Adding "now" emphasizes that you're playing cricket at the moment you speak. The shorter version would normally have essentially the same meaning.
In general, use of the present progressive doesn't imply an action ongoing at the moment you speak:
Q: What do you do with your weekends?
A: I'm playing cricket every Saturday this month.
So you can concoct situations where adding "now" is important to understanding:
Q: What are you doing tomorrow?
A: I'm playing cricket now. I'll answer you when the game's over.
(Without "now", it would mean you're playing cricket tomorrow)
The inclusion of the word "now" creates an implicit contrast with other occasions or with previous arrangements.
"I am playing cricket" could be used to describe a future arrangement or a habitual or repeated action:
I'm playing cricket next Saturday.
Every time he phones me I'm playing cricket!
However, "I am playing cricket now" doesn't necessarily that you are playing cricket right now at the same moment that you're speaking or writing. That is just one possible interpretation. Another is that you are still describing a future arrangement or habitual action, but drawing a contrast between the current understanding and a previous understanding or arrangement:
MIKE: Jim said he didn't want to play football on Saturday.
MARY: Oh. So what'll you do instead?
MIKE: We're playing cricket now (instead).
You know how I play football at school every Wednesday? That's been cancelled. I'm playing cricket now (instead) [on Wednesdays].