Will not Move when put into drive or reverse

I was driving to work this morning and when I accelerated after stopping at a stop light the van hesitated and jerked a little and then on down the road it kind of did it again. The more I drove the worse it got and by the time I got to work my van was hardly moving. When I parked it and put it into park I shifted it to reverse and it would not move and just the motor would rev and then I shifted it into drive and it did the same.