They are both top level domains.
.com is generic and is used worldwide while .com.ng is for Nigerians.
So you can understand better, .com.ng is Nigeria, .co.za is for South Africa, .co.uk is for Britain, .us is for United States and .ca is for Canada.
If your brandname is your domain name and is very important to you, you can buy both the .com and .com.ng versions.
One more thing, Google places more importance on .com.ng. (seriously, I researched).
Olawunmi,
Hostmot Sales Rep., (
www.hostmot.com)