Both are functions. Try not to get tripped up on notation of functions. They are they same, just written differently.
f(x) = 3x-10 is the same as 0=3x-10
2x+7y=28 is the same as 2x+7y-28=0
Just one will give you a line and the other will give you a circle. If you are ever unsure, plug them into a graphing calculator and see what you get back.