There are a number of different types of winter covers, each suitable for different situations and budgets.
Probably the cheapest is the water bag cover for in-ground pools. This consists of little more than a tarpaulin stretched over the pool, but not sagging in to it, and secured in place with water bags around the perimeter. This cover is cheap but unsatisfactory because it seldom stays where it should. These type of covers are often supplied with little or no drainage. Rainwater accumulates in the centre of the cover, until the weight causes it to collapse into the pool. It then takes all the built up rubbish with it.
Mesh covers are a much better alternative. This is also stretched over the top of the pool, but this time is held in place by stainless steel fittings which are drilled into the pool surround and attached to the cover with straps. These fixings make for a much more secure cover. The mesh effectively acts in a similar way to a filter. The water is allowed through, but the dirt remains on the top. The mesh cover can be tailored to fit different pool shapes. The spacings between the straps will depend on the size and shape of the cover. Generally the larger and more complex the shape of the cover then the more straps it requires. Consequently they are more expensive, per square foot.
Covers which are commonly called solid covers are in fact large sheets of industrial PVC. These are usually secured either with stainless steel fittings in the same way as the mesh cover, or with aluminium bars. These are very effective and often sold as safety covers, as access to the pool is very difficult. Unfortunately this same access problem makes the cover difficult to move when covering and uncovering the pool. These covers are much more expensive than the traditional mesh ones.
Genuinely solid covers are available which are made using a slatted, rigid PVC. These covers float on the surface of the water, creating an impermeable seal, and will take the weight of an adult. Although these covers are by far the most expensive option, they are safety covers that can be used during the summer too.
Whichever cover you opt for it is important to measure the pool correctly. The manufacturer needs to know the exact pool size in order to allow for a good overlap. This prevents dirt from blowing in underneath. Most winter covers have special hard wearing protection sewn in to the cover at certain points. The manufacturer needs to know the pool dimensions in order to position these correctly. The slatted covers must fit the pool exactly. If it is too big it will not fit between the pool walls, and if too small will not be supported correctly on the edges.