Updated: Dec 3, 2019
I'm thinking about a way to keep this post short...and...it won't happen. So sit back, relax, grab your favorite drink, and enjoy the course!
What you see above is a well-outlined, but very basic explanation, of the different types of string. Most people don't know where to start when it comes to string choices. This is completely understandable, considering the amount of string education out there is slim to none. Most pro shops fail to ask the right questions when determining a new string for a client. I'm going to do my best to give the rundown on the different types of strings and what they do best (as well as the trade-offs with each).
How about we tackle Kevlar first...sound familiar? It probably does because it's what many bulletproof vests are made out of! My elbow is hurting just thinking about that... Kevlar (Aramid fibers: another term) is rather antiquated at this point in time as a material for producing tennis strings. It is incredibly durable and controllable, but these features can be found in stiffer polys. To prevent elbow/forearm/wrist issues, Kevlar should honestly be avoided as an option. To put this into perspective, a mid-range stiffness for polys as a whole would come in around 210-220. Kevlar comes in around 600-700!!!
Some of the mid-range polys aren't soft enough for some people so 3x that isn't going to fly. That's why I recommend COMPLETELY STAYING AWAY from Kevlar.
Unlike Kevlar, there are some demographics who may enjoy stiff polys. Although I personally don't carry any strings stiffer than 215, many big hitters have latched on to stiffer polys, such as RPM Blast and Luxilon 4G. RPM Blast actually was the best selling string a couple years ago, beating out the all time champ: Prince Syn Gut Duraflex. That specific fact brings about my major issue with stiff polys: Most people should NOT be playing with them! These polys are made for big hitters (5.0+ level trained athletes) who need both durability and control; and cannot get these qualities from softer polys. These players typically don't suffer from tennis elbow, etc. because they have a specific, rigorous training program. Overall, consult your local USRSA certified stringer before stringing with any stiff poly.
This is where I live on the string spectrum personally. Soft poly (or hybrids) is my bread and butter for 3.5+ older junior to adult players, seeking a balance of feel and control. Durability in this category would be obtained by going thicker in gauge, but with a slight loss in spin potential. These polys fall in the under 200 range, although I typically shoot for under 190. For example, the string that I personally use (I'm a 5.0 player who has suffered from tennis elbow in the past) comes in at 182 (Diadem Solstice Power 1.20). The market has shifted towards these types of strings in the past 5 or so years and there is a very broad spectrum of them as a result. Therefore, consult your local certified stringer to see what strings would best fit your individual game.
Hybrid string jobs are exactly what you think they would be: One string in the mains, and another in the crosses. You can honestly hybrid anything together but hybrids are typically a poly (soft or stiff) and a multifilament, synthetic gut, or natural gut. This helps soften the string bed for a player who would like the control of a polyester, but their arm says "I don't think so". A hybrid job could also be used for a player who likes the feel of a multi or natural gut, but would like to maintain greater control with a poly. These hybrids can also go either way; meaning the poly doesn't always have to go in the mains. The player is going to feel the string in the mains more so than the string in the crosses, so those wanting a hybrid with natural gut often put it in the mains. Examples of this would be Roger Federer or Novak Djokovic (Nat. Gut / ALU Power Rough). Then you have some who go the other way like Andy Murray (ALU Power / Nat. Gut). Honestly, it all comes down to personal preference and what you want to feel more of! I could go on all day but let's move on to multis!
Multifilament string is not a "one size fits all" grouping by any means. Every string company has their signature multi, with specific technology, fiber number, wraps, etc. Multis as a whole are strings that are constructed by weaving many little fibers (multi...filament: Ah it makes sense now!) together into one, and then typically providing that with an outer coating. What we end up with is a very soft string with excellent energy return (power) and feel. These multis are best suited for any player looking for a bit more "pop" and touch in their game. Some multis are more controllable than others, but those seeking control normally hybrid a poly with a multi. I highly recommend multis for clients suffering with lateral epicondylitis (tennis elbow) or wrist issues; at least until their pain subsides and they regain strength in those areas. Lastly, there are a couple strings that have come out recently that fit the multi description but are actually monofilaments by nature. These are typically produced with the intention of being used as cross strings in a hybrid: Think durable and slick (for snapback i.e. spin potential). Overall these strings typically fall within the 120-180 stiffness range.