What do usability activities consist of?

What do usability activities consist of? Examples of past user experience tasks include:

• Register for an account on a client site, log in, fill in your profile and add a friend.
• Pretend you are looking for a pair of jeans, search for jeans, look through the results and proceed to checkout, but stop before the order is confirmed.
• Look around ABC site, wherever your interests take you and give us feedback on the look and feel, concept and overall layout.
• Compare a home page to a competitor’s (or industry leader’s) home page and tell us what we do better or worse than them and which site you’d choose to visit and why.
• Compare the purchasing/checkout function of two sites.
• Conduct a search for an item/site.
• React to a series of advertisements, video or static images.
• Answer a series of questions.