Hi Fred,
The proper voltage for any one individual emitter is usually somewhere around two or three volts DC. It will vary depending on the particular kind you buy. I think what you are looking at in the items above is collections of LEDs that have already been wired up in groups. For example, MR-16 is a type of small light bulb. A 5050 SMD strip would be a sort of ribbon with a string of 5050 type LEDs mounted on it in a long row. When a manufacturer groups them together like this they design the finished assembly to run at a particular voltage. At that voltage there is enough power to run all the emitters brightly, but not so much as to burn them out. It is hard to just look at a finished assembly and guess how it should be driven. Your best bet is to ask the manufacturer or the person you get them from.
If you overdrive a string of LEDs what will usually happen is they flash once, brightly, and then they're dead. Underdriving them may make them run dim, or not come on, but does no harm. If you have a totally unknown unit you can try feeding it a little bit of power and slowly work your way up until they come on. If they never come on them either you're feeding them backwards (they have a positive and a negative side, and will not run if hooked up the wrong way) or the assembly was dead when you got it.
If you feed LEDs low voltage AC they may run, but will probably flicker.
