Straight answer, because this is about your money.
Unlimited
You get a daily allowance at full speed (for example 1 GB a day). After that you stay connected at 1 Mbps until the next day starts. That's fine for messages, maps and music. It's not enough for HD video.
Day Pass
Works the same way, but after the daily allowance the speed drops lower, such as 384 or 512 Kbps. Good for chat and maps, slow for anything else.
Fixed data plans
A set amount of data (say 5 GB) for a set number of days. When it's gone, it's gone, until you top up.
Every plan shows its after-cap speed on the destination page before you pay. Browse destinations.