What is the difference between the perfective forms скушать and покушать

Question

I see both forms being mentioned as perfective forms of кушать

Answer 1

Покушать (or поесть) means to have eaten some unspecified portion of meal, while скушать (or съесть) is for finishing a whole thing (meal or just dish). A pair of examples:

Вы хотите есть? -- Нет, спасибо. Я уже покушал. (i.e. I have eaten [something] already.)
А где мороженое? -- Я его уже скушал. (I ate it already)
Он съел свой обед (He has eaten/finished his lunch)
Попейте молока. Покушайте вот хлебца (Take some milk, eat some bread)

Also note that in Russian they more often use the verb form for "having a meal", e.g.: "(по)завтракать / (по)обедать / (по)ужинать" (to breakfast / to dine / to supper) vs. "съесть завтрак / обед / ужин" (to have breakfast / dinner / supper). Although in this context "съесть" doesn't necessarily imply eating meal a whole, yet it is less popular in speech and sounds a little bookish.