A correct way to look at the definition is with ?f
.
If you define f[x_]=x^2
(without colon) you will see the output straight away.
If you define f[x_]:=x^2
(with colon) there is no output.
In most common situations the input f[x]
will give the answer you expect, but not for example when f[x_]:=RandomReal[]
.
Look up SetDelayed
and Set
in the documentation.