The least expensive integrated amplifiers are receivers without tuners. Most of the Japanese made integrated amplifiers offer little or no sonic advantage over a receiver. It is possible to get very high quality sound from some integrated amplifiers. Integrated amplifiers typically offer smaller power supplies than separate amplifiers and use preamp components that don't quite match the best found in the highest quality preamplifiers. Separates Integrated amplifiers and receivers have their own switching facilities and volume controls.
Separates require an additional component to do switching and volume adjustment.
Generally speaking, separates represent the ultimate in amplification. There is a wide range of products sold in this category. There is no question that solid state equipment is more efficient, tests better and is more reliable. However, a large segment of the audiophile community say that tubes sound more lifelike and that transistors are too clinical. I find that this is more a political argument than a technical one.