While they are generally mostly the same, "Entschuldigen Sie, [bitte]..." is orders of magnitude more polite than "Entschuldigung...". You will normally use the former if you want something from the other person, such as when asking for the way or asking for a small favor.
The single word "Entschuldigung" is not truly impolite, but it is much closer to yelling "Hey, you!" at someone. Which is however still appropriate, for example when you address someone who just left the café and left his wallet on the table without noticing.
"Entschuldigung" is sometimes even used, especially by elder ladies, as in "Entschuldigung!" (note the exclamation mark) while they accidentially-deliberately hit you with their elbow or run you over with their trolley in the supermarket because they felt you were in their way.
Yet another shade of grey a bit darker, we get to "Entschuldigen Sie mal..." or "Entschuldige mal [... du Depp]", which best translates to "Hey, what the hell...". It's something you would use if, for example, someone took your spot in the waiting line while you didn't pay attention for a second.