This is actually a bigger question than what should be asked on a forum. You really need to talk to QAD or a consulting company and get some training.
However, I will try to answer this as simply as possible.
Discrete – Just a regular PO created as and when required.
Blanket – a “contract” with a supplier for a fixed amount but without due dates. Each time you call off from the blanket it creates a discrete order for the requested quantity and reduces the blanket by that amount
Schedule – Used for parts that you buy on a regular basis and is normally created directly from MRP. It also creates a cumulative amount which allows the system to receive more or less than required without needing to do multiple receipts. For example if I had two lines on a discrete for 100 each and I received a qty of 110, if I over received on the first line it would still show that they owe me 100, or I would have to receive 100 from the first line and the other 10 from the second line manually.