The backwards "K" is used to represent a strikeout when the batter does not swing at the final strike, used to differentiate between types of outs. The batter is considered to have "been caught looking," instead of swinging the bat at the final pitch of an at-bat.
The use of the "K" for strikeouts comes from a sports journalist named Henry Chadwick, who is credited with the creation of the standard scorecard system. The term "struck" was more common for strikes at the time, and since S was already used for a sacrifice play, Chadwick used the final letter K instead. Official Major League Baseball scoring registers any strikeout as SO. Kc and Ks are also frequently used to signify a called strikeout and a swinging strikeout respectively.