All commercial advertising time either available for sale or purchase from local TV stations. There are two major types of spot advertising:

(1)- Local Spot – Advertising bought on one station in one market. These sales are usually handled by the TV station.

(2)- National Spot – Advertising bought by national advertisers in several markets of their own selection. These sales are handled by the local TV station representatives.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf