what is this?

[Eric Price]

Hello;
I'm studying some code examples from the python cookbook site. I came across 
this:

def colsplit(l, cols):
    rows = len(l) / cols
    if len(l) % cols:
        rows += 1
    m = []
    for i in range(rows):
        m.append(l[i::rows])
    return m

What I'd like to know is what is the double colon? What does it do?
        m.append(l[i::rows])

Thanks,
Eric

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