Pliers are one of the most common
tools available to the
consumer or
professional. Pliers are used to grip or turn things, and operate by squeezing them closed in your hand. There are several different types of pliers but the most common are slip-joint pliers, linesman's pliers,
needlenose pliers and some people consider channel-locks to be a form of pliers. The most important job a set of pliers does is
grip. The two sections are the reason that a single tool is referred to as a "pair of pliers".
Slip-joint pliers are what most people think about when they use the word "pliers". Slip-joint pliers consist of two sections held together by an axle that shifts between two positions. The have a flat tip that closes completely in the outer position and remains open about 5mm in in the inner position. The flat outer section is good for gripping square or flat sections. Just inside is a larger rounded section good for gripping rounded things, like caps or pipe. The good thing about slip-joint pliers is that they are a fairly flexible tool, capable of holding items or turning most common nut sizes. The two axle-positions give them some adjustability, so items up to about 3/8" or 8mm can be gripped. They are also nigh-ubiquitous among homeowners. The disadvantage of slip joint pliers is simple, they don't do anything very well. The flat bit isn't shaped very well for gripping wire, and they don't have enough adjustment in gap to make them good for any serious gripping. It's very, very easy to round off the head of a bolt or nut using them because the size is almost always wrong. Don't use slip-joint pliers for anything that you really need to get tight. Channel locks or better yet a socket set work much better for those purposes. The virtue of slip-joint pliers is that almost everyone has a set. As a journeyman in the building trades I have a set or two myself, but hardly ever use them as I carry far more useful tools for the work I need to complete.
Linesman's pliers are a far more useful tool for the professional. Among electricians they may be the single most commonly used tool. A linesman's pliers is also two pieces with a pivoting on a central axle, but the axle is fixed in position. They begin a flat section with is machined with a cross-grain for twisting wires, and never quite closes completely. Immediately behind the flat piece is a wire-cutting section. Then you have the axle and behind it a section for gripping wire, metal fish tapes or a section designed for crimping.
With a set of linesman's an electricians can cut, strip and twist together wire if a splice is needed. They can be heavy. My set of Kleins (maker of the best of the type) is 9" (225mm) long, is made of tool steel and weighs over a kilo. Linesmans are often used as a light duty hammer. The gripping section behind the axle can be a very useful tool for pulling wire on long pulls. Linesman's are not very good for turning nuts or screws, but otherwise very flexible and excel at twisting wire. Every single electrician you ever meet carries a pair, and if he wants to travel light can often get by with linesmans and a screwdriver or two.
Needlenose pliers are another very common set of pliers. They are like linesman's in that they usually have a cutting section, but the flat section is long and slender, useful for reaching between small spaces to grab or pull something out. They come in many sizes from very small for fine or jeweler's work to fairly large for the building trades. Heating and air-conditioning techs use a specialized pair where the long nose may approach a half-meter in length! Needlenose pliers are carried primarily because you can use them to reach in and grab something otherwise hard or impossible to reach. Their purpose is gripping, though the small tip makes them useful for striking small targets. Shaogo told me his restaurant employs a pair of (sanitized) needlenose for deboning salmon on his sushi bar.
Channel-locks are used primarily for turning screws, nuts or bolts. Like all other pliers they come in two sections connected by a single axle. Like the slip -joint pliers they are adjustable, except that for a channel locks five or more positions are available. In addition, the flat sections are not straight in relation to the axle as in slip-joint and linesman's pliers, but rather angled to improve their ability to turn nuts. The gripping section is toothed, and may be straight for gripping flat objects or rounded for gripping round objects. Channel-locks come in all sizes. I have owned a set 100mm (4") in length up to 400mm (16") but most are about 200mm (8") in length. As a journeyman I own four pairs of different sizes.
Channel locks are not as good at turning bolts and nuts as a socket set or an open-end or box wrench. Careless workers may easily round off a bolt using them. Used with care that sort of damage is rare. What they are is flexible, so a set can tighten many kinds of bolts, and quite fast if used properly. To use a set of channels it is important to remember that the 'top' section is the one that should apply the most torque. That rule is often broken, but is best for both the tool and the object it is gripping or turning.
Pliers is not really a term that should be applied to one single tool. Rather pliers is a term which applies to a family of tools that grips objects between two metal sections connected by an axle. Like other tools they can be both specialized or flexible and it is best to get the right tool for the right job. In my opinion, skip the slip-joint pliers and purchase one set of linesman's, one needlenose and one medium-sized set of channel locks. With those three tools it is possible to do a great deal of work well.