An absolute xpath in HTML DOM starts with /html e.g.
/html/body/div[5]/div[2]/div/div[2]/div[2]/h2[1]
and a relative xpath finds the closed id to the dom element and generates xpath starting from that element e.g.
.//*[@id='answers']/h2[1]/a[1]
You can use firepath (firebug) for generating both types of xpaths
It won't make any difference which xpath you use in selenium, the former may be faster than the later one (but it won't be observable)
Absolute xpaths are prone to more regression as slight change in DOM makes them invalid or refer to a wrong element