Understanding "Leading/Lagging" and "Phase Shifting" with Sinusoidal Functions?!
You could think of leading and lagging as to whether a sinusoidal signal has a head start with respect to a reference wave, this would be the simplest way I could think of describing it. When you look at a cosine and sine wave the cosine leads by 90 degrees because, for this example cos(0)=1 while sin(0)=0. sine will not reach a value of 1 until a 1/4 of the cycle has occurred which is 90 degrees (360/4).

Has this helped?