The swivel is normally the tie point between line and leader. It is supposed to act, as its name implies, as a swivel point. Essentially, the leader and everything below the swivel can spin and twist with out twisting your line. Instead, the bottom half of the swivel is supposed to turn.
The problem here is that the vast majority of swivels being purchased today swivel very little, if any at all. At best they become a convenient splicing mechanism between line and leader. If you are using a swivel only for that purpose, save yourself some money and use one of many knots that will do the same thing with greater strength.
Inexpensive swivels meet the definition of a swivel, because they will turn in your fingers when you twist them. But when pressure is applied, turning them becomes difficult. These are the swivels most people buy and the ones that become simply a splicing point. They are generally made from twisted wire with a small barrel in the middle, hence the term barrel swivel. When the pressure of a fighting fish is applied, they actually turn very little.
Lures can also apply a lot of line pressure that prevents one of these swivels from turning. Have you ever fished a spoon using a cheap swivel only to see your line twisted to a fare thee well? Blame the non-swiveling swivel!
Good swivels are expensive. Ball bearings signify swivels that are built to do what the name implies. Even then, some good swivels are better than others. If you plan to fish a lure or bait that will twist your line, use a ball bearing swivel. They cost four times more than cheap swivels, but as my father always said you get what you pay for. Nothing can be more frustrating than line that twists so badly you can hardly cast you spinning outfit. A good swivel will take care of that problem.
So, if you are like many inshore anglers, using a swivel only as a tie point for a leader, try switching to a surgeons knot or blood knot to piece the two together. With a swivel, you have two knots to worry about. This way you have only one knot.
Offshore fishing almost demands that swivels be used - even if only for tie points. Heavy line and heavier leader material becomes extremely difficult to tie together, and a swivel makes that tie possible. If you fish offshore you know what I mean.
Most offshore anglers choose the more expensive ball bearing swivels for their terminal tackle. the risk of loosing a big fish is not worth the difference in price to upgrade to a ball bearing model.
If you do need a swivel, do yourself a favor go with a good ball bearing swivel. You wont regret it.