PLC's (programmable logic controller) are used to control machinery, usually in manufacturing and testing of products. They are usually programmed with some kind of "ladder logic" using brand specific development environments. The advantage to PLCs is ease of use. They come with boards that readily drive and interface to all kinds of sensors, (usually 4-20mA stuff). The disadvantages are limitation and cost. PLCs are usually a few hundred dollars for a decent one.
Compare this to a PIC and a handfull of parts for maybe $20. If you have the time this is a much cheaper and less restrictive option. I've been using PICs in production and test equipment for years, I've cut cost, improved flexibility and streamlined development. Another unexpected advantage turned out to be fear! The mechanics that take care of the production machines are afraid to touch my machines, this makes them last much longer!!