There are dozens of hanger types. Two standard widths used today are 3/8-inch hangers and 1/2-inch hangers. 3/8-inch hangers are ideal for clipping quickdraws
to, but are too thin to place ropes
directly through because they may cause core
damage when weighted. 1/2-inch hangers are used for anchors. They are designed so that they do not include any sharp edges. Due to the thickness and lack of sharp edges, placing a rope
directly through 1/2-inch bolts will not cause rope damage
(i.e. when cleaning a climb).
A popular alternative to 1/2-inch hangers are hangers with rings. When cleaning hangers with rings, you place the rope
directly through the ring (not through the hanger). Chains attached to bolts are also a popular alternative for anchors. Chains allow route setters to extend anchor points over sharp edges or high friction areas without damaging ropes. Always feed the rope
directly through the lowest link in each chain when cleaning. If you use a link that is not the lowest, you risk having two links 'cam' against a rope,
permanently jamming the rope.
Is a Bolt safe?
"It's often as easy to recognize good bolts as it is to recognize bad ones. Metolius hangers and Rawl five-piece bolts with a hex head of 1/2-inch across or larger are especially dependable." - The American Safe Climbing Association
There is an excellent article on this that you can check out here: The American Safe Climbing Association