If you want to carry your kayak or stand up paddleboard (SUP) on top of your car, there are several options.
For bare roof cars, a simple temporary carrier consists of 2 shaped foam blocks usually with anti-skid material on the bottom, 2 long straps, (a bow line and a stern line for kayaks). It is easier to have 2 people load a kayak, as the blocks are not held down and can shift position. The kit is used only when you are carrying your kayak, not left on your vehicle.
Another option for bare roof vehicles is the Malone Versa Rail, which has suction cups on the mounts and each load bar (front & rear) straps through the vehicle with the doors open, and the straps are then secured inside the vehicle with cam-buckles. You now have 2 load bars, to which anything can be attached.
Thule rack systems have an excellent reputation. They mount to bare roofs via a fit kit specifically made for the year, make & model of the car, a footpack which the fit kit attaches to, and a load bar in varying lengths, depending on the width of your roof. This system is considered more permanent; anything can be attached to the load bars, and people tend to leave them on the car year round.
For cars with side rails, both Thule and Malone make simple systems to attach to the rails that will provide you with sturdy crossbars. The Malone AirFlow bars provide both the mount and the bar together; the Thule Cross Roads is the piece that attaches to your side rails, then you purchase the load bar length that suits your vehicle.
Once you have your Load Bars you can attach kayak carriers, SUP carriers, bike carriers, ski carriers, etc. There is a separate blog on kayak carriers.