This is a fun one, today. The definitions of these words are related (they have the same origin), so that can add an extra dash of confusion into the already confusing world of homonyms.
Bases: more than one base
--> Base: the main component; the bottom foundation
Basis: the foundation; the main component
The difference is just one you'll have to remember: base is generally more literal (dirt with a clay base) and basis is generally more figurative (the basis of the argument).
For kicks and giggles, you can add into the mix the plural form of basis, which is spelled the same way as the plural form of base: bases. Words that become plural by changing -is to -es, however, tend to be pronounced differently (emphasis, ellipsis, etc.).
More than one base: bay-sehs
More than one basis: bay-sees
Please forgive the cooking talk (dash of confusion; add to the mix)—I'm in the middle of editing a cookbook and it looks as if it may be rubbing off on my writing! :)