Here is an image of the front of the right knee, opened from top to bottom and with the kneecap (patella) flipped over, showing its undersurface.
The image is illustrative, rather than fully accurate, as the knee really needs to be bent to allow one to look straight into the cavity of the knee joint like this.
See the shiny white cartilage that covers the end of the femur bone. As the knee bends and straightens this surface makes contact with the shiny white cartilage at the back of the patella (kneecap) as the two slide over one another. This contact between the femur and the patella is known as the patellofemoral joint.
The femur also makes contact with the top of the tibia bone, and this contact is called the tibiofemoral joint. It makes contact at the two rounded ends of the femur - the condyles. As the knee bends and straightens these two condyles roll over on the joint surface below.
Strong ligaments are present to stabilise the joint. In the image you can see the cruciate ligaments in the centre of the joint between the two condyles, holding the femur and tibia in relation to one another, but also allowing the joint to bend and straighten.
Sandwiched between the rounded condyles of the femur and the flattened top (plateau) of the tibia are the menisci - shock absorbers that fill the space and allow smooth movement of the joint.