A higher thread count simply means there are more threads per square inch, but what is agoodthread count depends on the style of the sheet. For example, good quality percale sheets usually have a thread count of 180 to 300, whereas good quality sateen usually have a thread count of 300 to 600. This is because sateen requires more threads, due to the weave style, than percale does. 

Polycotton

Ply refers to the number of fibers used to create one thread. One-ply sheets have one fiber, while two-ply sheets have two fibers wrapped together to create a single thread.