Natural Gut is the oldest type of tennis string and often the choice of professionals and advanced players. it offers an unrivalled combination of a lively feel and amazing power while holding tension perfectly over time. Natural gut is a completely natural product (cows intestines) which results in a higher cost. These strings can be less durable than other synthetics so if you are likely to break strings, alternatives should be advised.

Natural Gut pros:

Holds tension perfectly over time
Unrivalled power
Remains soft at high tensions
Superior comfort

Natural Gut cons:

Expensive
Less durable than other types of string
Doesn't perform as well in wet or humid weather

Multifilament string are the closest playing string to natural gut but for a lower cost. They are popular amongst both advanced and intermediate players as they can provide excellent comfort and power. Multifilament strings are usually more durable than natural gut but can lose tension more quickly. The strings are often made from braided nylon fibres and wrapped in a resistant cover.

Multifilament pros:

Closest string to matural gut
Excellent comfort
High elasticity
More powerful than other synthetic strings

Multifilament cons:

Although less expensive than Natural gut, still more expensive than other strings
Not as durable as other synthetic strings

Synthetic Gut strings are very popular with club players and are a great choice if you're unsure what to go for. Usually made from nylon and wrapped in flexible filaments, the construction offers all-round performance and a balance of playability and durability.

Synthetic Gut pros:

Well priced
Can offer different playing styles (from construction angles and materials)
Great all-round string

Synthetic Gut cons:

Does not hold tension as long as other strings

Polyester/Monofilament strings are constructed with a single polyester fibre and a thin coating, they are produced in different gauges which can enhance different playing techniques such as topspin. This type of string is extremely popular with tennis professionals as they provide the optimum level of power, control and spin.

Polyester strings would not suit every player. The last of elasticity and stiffer feel provides less comfort and may hurt the arm over time. They tend to lose tension very quickly in comparison with other types of string which results in loss of control so your racket my need to be restrung more frequently.

Polyester/Monofilament pros:

Extremely durable
Well priced
Produced in different gauges
Can provide optimum level of power, control and spin

Polyester/Monofilament cons:

Not suitable for players wanting comfort due to lack of elasticity
Loses tension quickly

Co-Poly (or Co-Polyester) strings have been deveoped to aid some the weaknesses seen in Polyester/Monofilament strings. As a result, they have increased elasticity and are able to hold tension for longer. This means that this type of string is a great choice for those seeking the power and durability of a polyester string but with a softer feel.

Co-Polyester pros:

Similar qualities to polyester string but much softer on the arm
Holds tension for longer then polyester strings

Co-Polyester cons:

Not usually as durable as normal polyester strings

Hybrid strings are where the racket is strung with two completely different strings in the vertical mains and the horizontal crosses. This can be as simple as using different gauges (thickness) of the same string but more often, different types of string are used; for example, a monofilament string in the mains and a multifilament string in the crosses. Hybrid strings allow the player huge flexibility when deciding on a string setup. Combining different strings can offer a better balance of the different characteristics of their chosen strings. Some of the more common reasons for choosing hybrid strings is to improve comfort or add durability.

There are pre-packaged strings that you can buy of more popular hybrid choices. For example, Luxilon Roland Garros Duo Power Tennis String 16G (12m set) which features Alu Power and NXT Power to create the ultimate power string.
For more flexibility, you can pick from individual string sets and combine them together to create the hybrid string most suitable for you.

Hybrid pros:

Offers huge flexibility to the player to balance certain qualities of strings, or enhance them, to best suit their playing style.

Hybrid cons:

Finding the perfect combination of strings to suit a playing style may take time and be trial and error.