fundamental difference between Primary/Clustered and Secondary/Non-Clustered index
And it looks like curiosity didn't actually kill the cat :-). So I found this book Database System Concepts (Sixth Edition) online and it's freely available to be read. I've scratched it a bit and it turns out that it has the needed definition inside:

Primary index: in a sequentially ordered file, the index whose search
key specifies the sequential order of the file. Also called
clustering index. The search key of a primary index is usually but
not necessarily the  primary key.
Secondary index: an index whose search key specifies an order  different from the sequential order of the file. Also called  non-clustering index.

So there you have it. Now I suppose you were reading another book (by same author(s)) which was with tests inside (Q&amp;A) and not this one with theory. I'd say you should use them both at the same time.
Now, I'm not sure that a general book would be the best way to start. Personally I'd start with any freely available kit (SQL Server Express, MySQL Lamp, Oracle XE, SQLite...whatever sounds good for your ear) and play with concepts and also their physical implementation. I'd say going also with examples will work better than just pure theory.