Molecular Biology of the Cell 5th Edition (2007) pgs 1464 Alberts et.al.
Myoblast cells are progenitor cells that form muscle cells. They can be considered as the precursors to skeletal fibres. Special proteins expressed by specific genes help in the specific proliferation of myoblast cells. Various myoblast cells fuse together to form a bundle of cells with a specific function. Thus forming multi-nucleated skeletal muscles fibres. Myoblast cells donot fuse to form smooth or cardiac muscle cells.
Satellite cells are remaining, mononucleated myoblasts found in adult life which don't fuse with the muscle fibre. Unlike muscle fibres, the are able to under go cell division by mitosis. Therefore, they are often referred to as myoblast reserves and they are able to fuse with the muscle fibre when it needs to be repaired or during hypertrophy (increasing the size of a muscle fibre). Astrand, Per-Olof., et al. (2003). Textbook of Work Physiology Physiological Bases of Exercise. 4th ed. United States of America: McGraw-Hill. p31.