The workhorse of the forest is the tracked vehicle known generically as "a Cat", no matter whether it is made by Caterpillar or not. The left picture shows a Fiat-Allis earthmover working in a quarry, while the right image shows a Caterpillar 527 tracked skidder. The Fiat-Allis model is quite a bit heavier and larger than the 527. Note the grapple and moveable arch on the 527.
Also note the high roller style track design on the 527. This style of track design allows for less wear due to dirt, since the roller is kept out of the near-ground environment and the roller can be positioned forward/rearward along the vehicle axis during fabrication so as to provide a balance and traction combination for skidding. The same frame and chasiss can used to build a Cat D-5 earthmoving Cat and the high roller position and track length adjusted for that use.
Both machines have blades for moving earth and rock. The blades on the front of the tractors can be straight, angled, or U-shaped depending on the major work to be done. The Fiat-Allis also has large downward facing toothed blades called rippers on its rear end. These are used to shatter rock, The smaller blade on the 527 is used to clear skidding trails, pile brush, clear landings of debris, and push stuck trucks, among many other jobs needed on a logging site.
For more good information on "cats", go to this link: