What is non clustered index and what is B-tree structre in index.

Question

what is non clustered index. can i make non clustered index using 2 table of 4 columns. For example 2 table is first is Customer and second is Employee table. 4 colums from both table like cust_name, cust_id, emp_id, emp_name. can i make non clustered index using these columns.