You collect power ups they increase your weapon, die and they decrease.
I've improved the control in v1.8 (should be on the store within a few days), I've dampened turning down and restricted the thrust to only occur if you are pressing up...
If you survive a wave with all intact you are rewarded with a bonus, possibly also an extra life. You should see them appear inbetween waves.
I've changed the description info based on your feedback to give clues as to what is happening.
I might add an arcade 'attract' mode that shows baddies, number of points, etc - then gives a demo of playing. What do you think?