The official currency of Angola is the Kwanza, available in note and coin form (you'll seldom see coins though)
Notes in circulation are 5,10,20,50,100,200,500,1000,2000,5000

As a rough guide the exhange rate is approximately 95kw to the dollar (150kw to the pound)

Luanda is mainly a dollar economy, with almost everywhere accepting payment in dollars (many restaraunts your bill will have both amounts written on it)
Changing money can be done in many shops (pay in dollars, change in Kw) or by using one of the many licensed money changers in the street (these are usually women and they'll have an ID document around their necks)
You can change money in the banks, but you'll get a lower rate.

There is little in the way of an international banking infrastructure in Angola, whilst there are ATM's in Luanda, they don't accept foreign cards and with the exception of a couple of hotels, foreign visa cards aren't accepted anywhere either.
This is a cash economy, bankers drafts aren't in use and whilst Western Union can transfer money into Luanda, the service can be slow.

Do not carry all your cash around with you, and separate what you do carry between various pockets.