:global

:s (:substitute) is one of the first commands any Vim user learns. There are several similar commands that are more generic. Like :s, these commands can take a range; however, unlike :s, there is no % variant, as these commands are global by nature.
These commands can be seen as a simpler, less general-purpose alternative to macros that takes less keypresses.

Commands

:g (:global)

Instead of substituting strings matching the first part with the second, runs the (command mode) command on strings matching the first part.

:v (:vglobal) (equivalent to :g!)

Same as :g, but targets strings that do NOT match.

Use cases

Delete lines containing pattern

g/pattern/d
Practical usage:
g/TODO:/d
g/debugger/d
g/binding.pry/d
Optimization: g/pattern/_d puts deleted lines in the null register, which has no performance cost.

Conditional find/replace

g/pattern/s/word/newword
This "scopes" the substitution to matching lines (for example, to scope your substitution to comments).

Yank all matches

g/pattern/y A
Note that we are yanking to an uppercase register, meaning we append matches. If we did y a, each yank would overwrite the previous yank.

:norm

g/pattern/norm ^ihello
This applies the normal mode instruction (here, "go to start and insert hello") to matching lines.
Practical usage:
g/debugger/norm gcc
g/pattern/norm @q

:exe

g/pattern/exec "norm!" "A\r"
When using :norm, you might notice that you can't insert linebreaks -- and <CR> is literally printed as the strings "\r" and "". :exec(ute) lets us actually pass in commands as strings to be executed, like eval().
This is a somewhat meta pattern that follows the last example. It executes the two separate commands "norm!" and "A\r" to the pattern. Before execution, the second is evaluated to "append carriage return."