what is the most lightweight oracle version?

I guess it will depend on the sorts of things you are wanting to learn.

I've been using Oracle Database 10g Express Edition as a datastore for personal/learning software development projects. From Oracle's website:

Oracle Database 10g Express Edition (Oracle Database XE) is an entry-level, small-footprint database based on the Oracle Database 10g Release 2 code base that's free to develop, deploy, and distribute; fast to download; and simple to administer.

For what I need, I've found it quick and easy to use.

Oracle XE is the Oracle Database 10g Express Edition.

If you don't need any special 11g features though, this might well be sufficient for you.

yes oracle express edition is the one you want if you have a weaker computer. it requires 512 MB RAM to work smoothly. however, it is based on Oracle 10g release 2, so it is not quite 11g. but since you are just learning it is good. start with the tutorial here and other books in the oracle site.

http://st-curriculum.oracle.com/tutorial/DBXETutorial/index.htm