Basically what Cireon says, and thank you from me as well!
Regarding your questions,
The burst upgrade increases the number of projectiles a weapon shoots at the same time.
That would for example be more beams for the laser, and more missiles for that weapon.
The green power ups are permanent, so definitely worth getting. The gain for each little orb is relatively small, so missing a few here and there will not ruin your run completely, but they do add up exponentially right now, so you don't want to miss too many either.
Regarding the yellow power ups, I challenge you find out for yourself!
If you have not seen it, notice the little bar that appears at the top of the screen once the first yellow orbs appear, and try to fill it.
Consider it a sort of ultimate challenge of the game as it is right now.
Thanks again, and glad you like it!