In terms of analogue video signals and picture quality...
Component > RGB SCART > S-Video > Composite > RF aerial
Component - 3 colour RCA plugs (Red Green Blue)
RGB SCART - 21 pin SCART plug
S-Video - either SCART plug or dedicated 4 pin din connector
Composite - either SCART plug or single yellow RCA plug
RF aerial - two core COAX used for your aerial cable
Component can also be interlaced or progressive but it depends on the quality of your DVD player and Plasma's de-interlacer as to whether progressive scan would give a better picture in your case or not. Probably best to try it out and see, some see vast improvement, others minor improvement whilst some see a better picture on interlaced component.
Try to use Component or RGB SCART only if possible since they are significantly better than the other options. Steer clear of composite, utter pants.
S.