Usually power output, engravers run below 80 watts and cutting tubes are usually 80 watts+. Both will cut but tubes over 80 watts have a minimum strike power below which they will not lase.(engraving is done at low power, typically under 15% so making high power tubes too powerful for engraving).
CO2 gas laser tubes come in several types,
Sealed Glass, Sealed RF, Boxed Glass are just three of them.In function there is no real difference (they all produce invisible IR laser beams) in function but price they go from $150 to just over $35,000.
If you want to build a laser machine the parts are readily available off the shelf, if you just want to build a lasing box then while pretty easy it's also dangerous. The wavelength of the lasers they produce is invisible, (no pretty beam shots) and they run typically (in glass DC tubes) from 16Kv upwards (around 40Kv being the max).
While the amperage is low, in a loop across your heart they will throw your ticker into cardiac arrhythmia , unless you get some pretty quick help from a medical professional (like within 3-5 minutes or so) then I'm afraid it's lights out.
Big numbers tubes also have a sometimes unexpected ability to set shit on fire...very quickly and at quite some distance. Just because you can't see the beam doesn't mean it's not dangerous.
Before even thinking about a tube buy some laser goggles rated for IR in the correct wavelength (10600). NEVER power up an exposed tube unless you are wearing them. Remember these things aren't pointers, they aren't toys, and they never forgive mistakes.
cheers
Dave