:g, short for global, executes a command on all lines that match a regex:
:g/LinesThatMatchThisRegex/ExecuteThisCommand
Example:
:g/hello/d
This will delete (d) all lines that contain hello.
On the other hand, :%s just performs a search (on a regex) and replace throughout the file:
:%s/hello/world/g
The g at the end means global or greedy (this is disputed) so it will replace all occurrences on the line, not just one per line. You can also use the c flag (:%s/hello/world/gc) if you want to confirm each replacement manually.
This command replaces all occurrences of hello with world.
Both the :g and :%s commands support regular expressions.
The s command means substitute and the % means throughout the buffer. So %s means substitute throughout the entire buffer. You can also give a line range:
:10,15s/hello/world/g
This will execute the search and replace seen earlier on only lines 10 to 15 (inclusive).