Sample-Drawing Procedure

Sampling scheme is a collection of sampling plans that you define depending on your business requirement. For example, for a material your existing legacy sampling is: Upto 100kg : 1kg sample, 101-200kg ; 1.7kg sample, and above that 5 kg sample. so you see multiple sampling plans are associated with the same material and you want SAP to auto determine which sampleing plan to use based on your lot size. So you use a sampling scheme where you define all these plans in one table and also provide acceptance/ rejection criteria.

Sample drawing procedure defines how these samples are to be collected, like, what type of samples (Primary, pooled or reserve), no. of samples to be collected, container type, schemes etc.

Read the documentatuion part of the document I gave you, you will understand the basics.