Scratch

Variables are simply values; they can't create blocks 

Broadcasts are local to each Scratch project (unless you use Mesh), and all sprites and the Stage will receive them.

broadcasting is way of making other things do something

like lets say when a sprite moves you want the other sprite to do something
broadcast "whatever" and then for the other sprite when i receive "whatever" do this

for variables those are numbers.
a good example would be health.

and mesh is a way of connecting two local ips to run a project simultaneously.

Variables
One use for a variable is to keep track of lives. You could put this script:

When green flag clicked
Set 'lives' to 5
Forever
If touching enemy
Change 'lives' by -1
Wait until not touching enemy
End if
If lives=0 or lives<0
Stop all
End if
End forever

Once you get the hang of it, you can change the stop all bit to a play again screen or something like that.

Broadcasts
Broadcasts send a message to all other sprites, telling them to do something.
If you had two sprites, and you wanted sprite2 to move, but only when sprite1 is clicked, then you would put this script on sprite1:

When sprite1 clicked
Broadcast move
Move 10 steps

And then this scripts on sprite 2:

When I receive move
Move 10 steps

Hope I helped!

A broadcast can't be "seen"; only sensed by the program.
From there you could have the program do something though.