The text below is from 2013, and is outdated. Opening links in new tabs can now be turned on in the user preferences.
This is a tough decision in web design, because different people prefer different things.
Most usability experts recommend not using target="_blank" because it overrides user preferences. There is no consistency across the web about how websites do that so one never knows what will happen until clicking.
I prefer to leave the decision up to each user:
If you want to open a new tab you can hold the ctrl key down when clicking. That opens a background tab in Firefox or Chrome. If you prefer foreground tabs instead of background tabs, you can change that in the browser settings.
If you prefer a link to open in a new window, you can hold down the shift key when clicking on the link.
If you accidentally close a tab, press ctrl-shift-t.
If you accidentally close a window, press ctrl-shift-n. (Firefox only)
To move between tabs use ctrl-tab and ctrl-shift-tab on the keyboard.
To close a tab, press ctrl-w.
If you use those shortcuts, every link on every website will open in a predictable way.
If I force the links to open in a new window, there is no way for users who prefer links to open in the same window to override the new window, while if I leave it like this, people can use ctrl or shift when clicking to open the links in their preferred way.