Many people associate strikes with picket lines. Workers bearing placards with slogans supporting their position file around the gate of their workplace, often chanting or even singing songs. The purpose of picketing is to draw public attention (and sympathy) to their cause, inform the public of the goals and the reasons behind the strike and discourage anyone from violating the strike order and going to work. Anyone who does this literally has to cross the picket lines, and they usually are called scabs.
Scabs can be union members who decide to work instead of striking, or they can be non-union workers specially hired by the employer to fill the positions of the striking workers. The term goes back to the 18th century, and probably refers to diseases common in that era that left victims with infectious scabs. Workers crossing picket lines would be "scabb'd" by the other union members (shunned and forced out of their jobs).
In 1806, the Cordwainers Union of Philadelphia went on strike, and several union members became scabs. This was recounted in the ensuing trial by cordwainer Job Harrison:
Sometimes union members will picket without striking. This is known as informational picketing. Prior to the air traffic controllers' strike in 1981, the controllers picketed outside airports during their off hours, holding signs and handing out pamphlets that explained their position [ref].