Live-action refers to television programs that use living actors as part of their cast. Zombies need not apply. Most television programs fall under this category. Shows that are not live-action programs include animated features, computer rendered programs as well as stop-motion or claymation shows.
The term likely originated with the notion of sitcoms being filmed in front of a live studio audience. This practice was popular during the 1970s and early 1980s, but is not really utilized any longer. Perhaps the actors got tired of audience members chucking beer cans at them after a bad performance. Who knows?