Skype is great. It's free to call somebody else's computer from your computer, and you can talk for as long as you like. Video is great because it's very often easier to understand what someone is saying if you can see their mouth movements and facial expression. You can also type into the Instant Message window while you're talking if you need to share text or clarify what's being said.

A Skype user is identified by their id (a unique identifier), but they can also set their name (which is of course not unique). If you want to connect with someone, you can use Skype to search for their id or their name. Searching in Skype is one of its weakest features. I nearly always see a long list of search results even if I search using the id. Then I have to right-click each search result in turn, view profile, and see whether the id matches what I searched for.