A conventional deadlift has more knee bend and lower hips in the down position, and involves more midback in the pull.
A RDL keeps the knees at a slight bend throughout the movement, putting more emphasis on the glutes and less in the back during the pull. RDLs usually don't go all the way to the floor on each rep.
Here's a pretty good looking conventional dead (not the best [I'd want him to have his shoulders over the bar more in his start position], but it gets the idea across):
conventional deadlift
Here's a Romanian Deadlift that looks pretty good:
RDL
Does that help?
Lisa~