Number of subsets of length 7 I have the following summation:
$$\sum\limits_{k=7}^{n}  {k-1\choose 6} $$
and apparently it counts the number of subsets of {1, 2, . . . , n} having size 7.
Why is this? 

You probably know that ${n\choose 7}$ is the number of subsets of $\{1,2,\ldots,n\}$ of size $7$.
Each such subset has a largest element. The largest element must be one of $7,8,\ldots,n$. For each possible largest element, let's count the number of subsets having that as a largest element. We already chose the largest, so there are $6$ elements left to choose. So we add all the subsets of size $7$ with the largest element $7$: choose $6$ elements from $\{1,2,3,4,5,6\}$, that is ${6\choose 6}$. Then the largest element $8$: that is ${7\choose 6}$, and so on.
The final count of the subsets is
$${6\choose 6}+{7\choose 6}+\cdots +{n-1\choose 6}$$
$$=\sum_{k=7}^{n}  {k-1\choose 6}$$
which is therefore equal to
$${n\choose 7}$$
