We have a voting system (note the arrows in the top right corner of each post) where you can vote on individual posts and influence the reputation of the poster. However, members can vote on posts because they agree with the post, disagree with the post, find it in poor taste, find it helpful, find it disgraceful, or for just about any reason. It's entirely subjective and meant to give a very broad overview of what the community thinks of a particular member, much like one's real world reputation.
Skill endorsements are meant to be a much more objective view of whether the particular member is skilled in a particular subject area. You can endorse a member by going to the Endorsements tab of their member profile, and clicking on 'Me'. You can also see all of a member's endorsements from that page.
The system will use your own history to come up with a custom generated list of who it suggests would be good people to endorse. You can view your list of custom suggestions by clicking on the Endorse link in the header of any page of the site.