In general terms, a recumbent bicycle is one where the rider sits in a seat, reclines against the seat back, and places the feet on pedals located in front of the seat area. Sort of like a Lazy-Boy® recliner on wheels.
The reclining angle of the seat and the position of the pedals in front of the rider can vary depending on the design. On some bicycles the steering comes from handle bars located about just chest high and other designs use two posts located on each side of the seat.
The term, recumbent, can be quite a mouthful so you will frequently here people refer to the bike affectionately as a bent, hence the name of my website: Going Bent.