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.
