In LaTeX, \sum is rendered as inside math mode. The difference between the summation operator \sum and \Sigma, which generates the capital letter , is that \sum appears larger, and that it supports the limits to be displayed below and above the symbol.
The following example illustrates the difference between \sum and \Sigma.
 Markup \sum_{n=1}^\infty\frac{1}{2^n}=1  Renders as
 Markup \Sigma_{n=1}^\infty\frac{1}{2^n}=1  Renders as