There are two general categories of backstay: The permanent backstay is attached to the top of the mast. The running backstay is attached about two-thirds of the way up the mast (sometimes at multiple locations along the length of the mast). In general, most modern sailboats will have a permanent backstay and some will have a permanent backstay combined with a running backstay. Backstays are not always found on all vessels, especially smaller ones.
A permanent backstay is attached at the top of the mast and may or may not be readily adjustable. In a mast head rig, tensioning the permanent backstay will directly tension the forestay. This control is used to adjust the amount of "sag" in the headsail. In a fractional rig, tensioning the permanent backstay will have two effects: First, the forestay is tensioned (controls sag in headsail) and second, the mast bend is increased, particularly in the upper one-half to one-third of the mast. Increased mast bend tends to reduce the draft (camber) of the mainsail.
A running backstay always attaches to the mast at a point below the top of the mast and is generally used in conjunction with a permanent backstay. Running backstays are found on both masthead rigs and fractional rigs. There are some rigs for which running backstays may be used without a permanent backstay. This occurs most often where the mainsail has significant roach; here, a permanent backstay would interfere with the operation of the mainsail. As a direct consequence of its attachment point (below the top of the mast) a running backstay is always adjustable because it must be manually engaged and disengaged during every tack or jibe. Adjusting the tension on the running backstay has two effects: First, the forestay is tensioned (controls sag in headsail) and Second, mast bend is reduced (the mast becomes straighter). The overall effect of tensioning the running backstay is a deeper mainsail (more camber) combined with a reduction in headsail sag. If the running backstays leads to the mast where the forestay attaches, the effect of tensioning them will be as follows. Again the forestay is tensioned reducing sag in the headsail and second, mast bend is increased with flattening of the mainsail as a result. Both effects are desirable as the wind increases.
On modern ocean going yachts, the backstay is also commonly used as an antenna for Marine SSB radios and/or an amateur radio. Now this is accomplished by placing structural backstay insulators at either end of the backstay.