Clock skew - the ideal and the real waveform have the same length (frequency), but the real one became slightly misaligned in time, there is a phase shift from the ideal (ideal == the reference waveform drawn on top)
Clock shrinkage - the ideal and the real waveform have the same length (frequency), but the duty factor is now different than 50%, in that pic the portion of level 1 became slightly shorter than in the ideal waveform on top
Clock jitter - each period is now slightly longer or short than the ideal waveform, the length on average remains the same, but there is a small random wiggling of the length (frequency) of the real waveform when compared with the ideal one on top