We can summarize the difference between basic and advanced courses as follows:
Basic 
Advanced 

Objective 
Increasing USACO score to 500 out of 1000 in that level 
Promotion to the next USACO level 
Problem difficulty 
Easy to medium USACO problems of that level 
Medium to hard USACO problems of that level 
Prerequisite 
Requires previous level course 
Requires the Basic course of that level 
USACO level 
Typically for new students or students who score below 400 out of 1000 in USACO contests of that level 
Typically for students who score above 400 out of 1000 in USACO contests of that level 