50% of any money you make as an AUTHOR (of posts/ comments) is paid in STEEM Dollars, you can withdraw that immediately.
50% of the money you make as an AUTHOR (of posts/ comments) is paid in STEEM Power, you can withdraw that (into STEEM) in weekly instalments over 2 years.
100% of the money you make as a CURATOR (of posts/ comments) is paid in STEEM Power.
If you don't power up your STEEM into STEEM Power it is liquid (you can withdraw immediately). However STEEM Power earns interest that counteracts STEEM's high inflation. So it is best not to hold STEEM for too longer, just use it to transfer and trade.
(NB - there more depth to this, and I encourage you to read more widely. However these basics should help get you started)
I originally posted this as comment however creating a Steemit-FAQ post so more people have visibility. Steemit probably need a FAQs section as the same questions keep popping up