f(x) = |x| is the modulus function.

or, f(x) = x , x > 0

               0 , x = 0

              -x . x < 0

Clearly now x can take any real value, so domain of function is all real values.

Now for any real x, f(x) is always positive so range of f(x) is [0,infinity).

It is quite clear from graph of modulus function also. ( See Below )