psu ratings question

Alright, i know that the PSU is a very important part of the computer. I understand what to seek in wattage and efficiency. But im curious as to what the the voltage ratings mean. I will see like +5v@50a or 12v @38a. What does 50a and 38a mean, and what is good or should be seeked.

 

a is the ampres used (power) so you can have a item that takes say 5v and uses 30 ampres more ampres = more power used
it can be converted into watts by this calculation
" voltage X ampers = watts" (5x30=150watts) so the more ampers used the more power is used
there is no good or should be seeked just that if you buy components to run from your PSU you should make sure that they dont excedd the recomeded ampers (wattage) as this will cause your system to crash .
power supplys are not dear so go get yourself a 400+ watt one if your building your own machine this should provide more than enough power.