There are three main types of gutter protection.
1: Mesh screens. These are basically metal screens that resemble what you see on wrought iron patio furniture. They are slightly flexible and fit on the gutters either by friction or by screws. This is by far the least effective system for protecting your gutters. I've seen a lot of these become loose over time or they are completely missing altogether due to high winds. When you have Maple trees that release those "helicopter" seed pods, the large end with the seed tends to get stuck in the screening and they won't come out unless you pull them out! So much for maintenance free! The will also let small leaves and other debris through which can clog the system.
2: Solid covers. These are the ones that are always advertised as being maintenance free, have no clog guarantees, etc. These are usually the most expensive of all the options. Basically, these are designed to let the leaves flow over the gutters while the water flows into the gutters. Sometimes they require new guttering to be installed, but not always. In theory, it's a good idea, but in practice things don't always work out as advertised. These systems are usually attached under the first row if shingles using nails or screws.
When they do clog, there is no easy way to find the problem since you can't see inside the gutters. Also, since they are so inaccessible, it is a great place for insects, such as wasps, hornets and bees to make their homes. Mosquitoes also love the moist environment caused by a clog. While many of the installers offer free cleaning, you only know there's a problem when it's too late! Why do you buy these again?
3: Foam Fillers. These are relatively new to the gutter protection market. The brand I know and have experience with is Gutter Filter, but there are others. They are an open cell foam insert that install inside your existing gutters. When correctly installed, they prevent leaves and debris (no matter what size) from getting inside the gutters. The foam is very porous and will let water pass through very easily. Sometimes leaves may sit on top of the foam, but eventually they will either wash off or blow off. Some of the inserts sit flat across the top of the gutters, others form a dome and rise above the height of the gutter slightly. I prefer the taller material since it allows less debris to collect.
Here is a picture of what the product looks like when viewed from above the gutter. Of course, this is just a sample, the real thing will not have any gaps and the end of the gutter. Also, there IS a screw bracket in this piece of gutter, it is completely hidden by the product. You'd have a similar appearance if your gutters have the large spikes.