Most horses do fine without blankets, as long as they have a shelter to get out of the weather and wind. Horses that might benefit from blanketing include older horses, young foals, sick or injured horses, horses that don't grow much of a winter coat (for whatever reason), or horses that are body clipped for shows.
Irish knit sheets, fleece sheets, cotton sheets, and coolers are used for after workouts, when the horse is sweaty. These help the horse dry quickly without getting too chilled. You take them off once the horse is cooled down and at least mostly dry.
Fleece & cotton sheets can also be used as "liners" under light weight blankets or rain sheets, or as "show sheets" to keep your horse clean in his stall or in the trailer.
Nylon sheets are used to keep wind and rain off the horse, when extra warmth isn't really needed. Some may be lined with mesh so the horse doesn't sweat too much under them, or lined with fleece or wool to provide just a tad bit of warmth and softness for the horse.
Cotton duck blankets are similar as nylon sheets, but they are a bit warmer and more breathable, but less water proof. A friend of mine in Michigan uses all Cotton Duck sheets for her horses. To make them water proof, she uses wood deack sealer on them, lol. Works like a charm and they are "sturdier" than nylon rain sheets, so they hold up better to pastured horses. They're also easier to repair if they are ripped.
Turnout Blankets are the most commonly purchased outdoor winter blanket for horses. You can get them in varying weights (light, medium, heavy) for different climates. It's best to find one that is 1,000 denier nylon or higher, for the most waterproof and rip-resistant. They usually have gussets in the shoulders as to not restrict the horse's movement.
Fly & protecive sheets are used in the summer to ward off flies and keep a horse's coat from fading. The lighter scrim sheets provide the least protection and tear up the easiest. The heavy coated mesh Kensignton protective sheets provide the most protection and are very long wearing, but they can be "hot" in climates that are really humid. They also work well over winter blankets as protection from rips and tears.
Okay, I think I covered all of them, lol. The point is, your horse doesn't REALLY need any fancy blankets if he's healthy, has a good winter coat, and has shelter when he's outside. I haven't bought a winter blanket in 12 years! I have one cotton sheet that my dog sleeps on more than my horse wears it, lol.
"To be an equestrian in the classical sense is not just to be a rider. It is a position in life." --Charles de Kunffy
Anti-Slaughter and Proud!