Magic circle (sometimes called a sliding ring or a sliding circle).
For video instructions on how to do a magic circle check out this video by Crochet Geek.
Start by making a loop and inserting your hook
Yarn over (YO) hook
Chain (Ch) (Ch 1 for a sc, 2 for an hdc, 3 for a dc, etc)
Then make your first st into the circle (I'm doing scs here)
hold on to magic circle with left hand to keep it open
pull up a lp
YO again and pull through two lps, first single crochet (sc) made
Start to pull in magic circle by pulling on short tail end, making sure not to squeeze the sts so tightly you can't insert your hook later
Before you tighten the circle all the way, insert hook in the top of the beginning st
(this is particularly important if you are doing scs, because they tend to disappear)
Sl st in top of st
Pull short end tail to make magic circle as tight as possible
Now you have a nice tight center.