What is the difference between a primary and a secondary market?

A primary market is the main market to which you are selling.
A secondary market is an additional market to which you are selling.  

Answer

A primary offering, such as with a corporate bond, means you are buying it directly from the issuer, at par value, usually. A secondary market is where you sell or buy existing issues. I.E. If you bought a bond last year, now need to get your principal, you can sell it in the secondary market. You may not get par value. If rates are up since you bought the bond, then you will likely have to sell it at a discount to be able to get rid of it. If rates have fallen since you bought it, you could get a premium for it..