What product of Avast were you interested in? Free, Pro, AIS?
This is a comparison chart of the Avast products http://www.avast.com/comparison-chart to help you understand them better.
What other security software do you currently have or did you have in the past on this machine including antivirus (AV), firewall (FW), and other security programs?
Yes, you can transfer your license to another machine if you need to do that in the future.
@ LanceLongsword
The longest current legit License period is for 3 years, so if someone is trying to sell
you for longer than that, it isn’t legit.
During the license term you get all Program updates, unlike some companies locking you to a particular product Version like 2010 or 2011, etc. Currently we are on avast 5.0, but 5.1 should hopefully be out soon and avast 6 is in development (there is no planed date for release), as and when that comes out if your license is still valid (in date) you would be able to update to that version free.