A gaseous sample is generally allowed to do only expansion/compression type of work against its surroundings. The work done in case of an irreversible expansion (in the intermediate stages of expansion / compression the states of gases are not defined). The work done can be calculated using 
 $dw=-P_{ext}dV$
While in case of reversible process, the work done can be calculated using $dw=-PdV$  where P is pressure of gas at some intermediate stage. For an isothermal reversible process, since $P=\frac{nRT}{V},$ so,
${\left(w_{rev}\right)}_{iso}=\int dw=-\underset{V_i}{\overset{V_f}{\int }}\frac{nRT}{V}.dV=-nRT\ln \left(\frac{V_f}{V_i}\right)$
Since, $dw=-PdV$ , so magnitude of work done can also be calculated by finding area under the PV curve of the reversible process in PV diagram.