Subliminal - Something that your senses pick up that you're not consciously aware of. Your subconscious mind gets the message, but your conscious mind does not, so you can't filter or judge it. If it's visual, it usually flashes on the screen too fast to realize you've seen it, and if it's a PD audio CD, often you hear ocean sounds, rain, or whatever with the message whispered at a slightly lower volume.
Paraliminal - Clearly audible. With the paraliminal CDs from Learning Strategies, you hear two voices saying different things at the same time, one in each ear when you're wearing headphones. You can choose to focus on either one or focus on neither one more than the other mostly hear both at the same time.
Binaural Beats - See
Wikipedia's article.
Isochronic Tones - I'm not sure.