What is the difference between block mountains and fold mountains?

Block mountains:

Block Mountains are created when large areas are broken and displaced vertically. The uplifted blocks are termed as horsts and the lowered blocks are called graben. The Rhine valley and the Vosges mountain in Europe are examples of such mountain systems. 

Fold mountains:

When two tectonic plates of the earth’s crust collide, they fold over each other and form mountains known as fold mountains. The Himalayan Mountains and the Alps are young fold mountains with rugged relief and high conical peaks. The Aravali range in India is one of the oldest fold mountain systems in the world. The range has considerably worn down due to the processes of erosion. The Appalachians in North America and the Ural mountains in Russia have rounded features and low elevation. They are very old fold mountains.

Further Reading:

• Difference Between Folding and Faulting
• Geomorphic Processes