I had heard it mentioned once or twice but never knew what it was until I looked over at Wikipedia for the answer. The article over there is pretty interesting, both with old timer and present-day anecdotes. Here's the link.
The Eephus pitch is thrown overhand like most pitches, but is characterized by an unusual high arcing trajectory and corresponding slow velocity, bearing more resemblance to a slow-pitch softball delivery than to a traditional baseball pitch. It is considered a trick pitch because, in comparison to normal baseball pitches (which run from 70 to 100 miles per hour), an Eephus pitch appears to move in slow motion. Hitters typically get very anxious, swing wildly, and ground out.
I thought this particular story was pretty interesting.
After appearing in over 300 major league games, Rip Sewell only gave up one career home run off the Eephus, to Ted Williams in the 1946 All-Star Game. Williams challenged Sewell to throw the Eephus. Sewell obliged, and Williams missed the pitch. However, Sewell then announced that he was going to throw the pitch again, and Williams clobbered it for a home run. Years later, however, Williams admitted that he had been running towards the pitcher’s mound as he hit the ball, and photographs reveal that he was in fact a few feet in front of the batter’s box when he made contact (which would have resulted in an out if it was spotted by the umpire).