Windowing is when you multiply a Window Function by a Signal to extract a finite “window” of it.
Window Function
A window function is the function you multiply by the function of the signal to yield a “window” of the signal.
Rectangle Window
When you use rect as the window function, you get an exact subsection of the original signal. However, you also get Spectral Leakage due to the abrupt cutoff of the function. The other window functions solve this by tapering the product to zero by the edges.
Hann Window
Hann window uses a raised cosine shape that tapers smoothly to exactly zero at the edges to reduce Spectral Leakage.
Hamming Window
The Hamming window is a slight modification of the Hann Window optimized to minimize the height of the maximum side-lobe. Instead of tapering all the way to zero, it tapers to a small non-zero value.
Blackman Window
The Blackman Window is the smoother version of the Hann Window, achieving this smoother taper through an additional cosine term.