Power Spectrum Estimation of three sinusoids in white noise
The standard Periodogram uses all the data and computes its power spectrum spectrum estimatation at once; therefore it provides more spectral resolution but less estimation reliability (larger estimation variance).

The Welch's modification to the standard Periodogram is the concept of dividing the signal into shorter blocks and averaging the computed per block Periodograms, and therefore it has less spectral resolution (due to shorter blocks) but provides smaller estimation variance, due to averaging.