Strings are commonly grouped in four different categories; Natural Gut, Multifilament, Monofilament, and Polyester. Natural Gut will provide players with the most feel and touch, while Polyester will provide players with the most durability. Multifilament and Monofilament are made from synthetic gut material and provide players with a fair balance of both feel and durability.

Natural Gut: The Rolls Royce of tennis strings, best of the best, top of the line tension maintenance and feel. Most common among pro players and club level players, downsides include high price and being weather sensitive.  
Ex: Babolat VS Natural Gut, Prince Natural Gut

 

Multifilament Synthetic Gut-Multifilament is a coreless string with multiple synthetic fibers twisted together similar to natural gut. Advantages include increased playability and feel over a monofilament string.
Ex: Wilson NXT ,Tecnifibre X-One Biphase

Monofilament Synthetic Gut-The most popular synthetic on the market, they derive durability and tension retention properties from their solid center cores. An outer wrap of smaller filaments or fibers assist in tertaining tension and protect the core from small nicks and abrasions. The number & construction of wrapped filaments, diameters, and blends vary from string to string.
Ex: Babolat Super Fine Play, Prince Synthetic Gut Original

Polyester: Offer players increased durability over synthetic gut and Mono/Multifilament strings. Popular among hard hitters who break strings often noted for giving a dead feel and being very stiff for some, most common among tournament players and pros. Kevlar falls into this category as well.
Ex: Luxilon Big Banger ALU Power, Babolat Pro Hurricane Tour

 

* Side note Hybrids: Hybrid strings consist usually of a durable string in the mains (poly) and highly elastic synthetic strings or natural gut strings are used to provide comfort and feel in the crosses. Hybrids provide good playing characteristics while a poly/multi hybrid often lasts longer than a pure poly or pure multifilament string job. Most common among tournament players and pros, as a side note, Roger Federer strings his racquet with the polyester in the crosses and natural gut in the mains.
Ex: Babolat Pro Hurricane Tour/VS Natural Gut, Kirschbaum Pro Line II

 

String Tension	Power	Control	Durability	Feel	Comfort
softer	more	less	less	more	more
tighter	less	more	more	less	less

 

String Gauge	Elasticity	Durability	Spin	Feel	Comfort
thinner	more	less	more	more	more
thicker	less	more	less	less	less