The institutional investors

These are typically organizations which have access to large pool of cash. These guys use the money to invest in stock markets/ companies.

Given the sheer volumes they deal with, they have the ability to move the markets. For example, if LIC (Life Insurance Corporation) purchases 10 lakh shares of Tata Motors in the market, the price of Tata Motors tends to move up (the economics funda of demand being greater than supply).

Apart from mere volumes, the one other reason why they generally influence markets is because of "signalling" effect. These guys are sophisticated investors (at least they are supposed to be). So, their actions are very closely watched and interpreted by almost all the market participants and thus serves as a signal (either positive or negative).

Institutional investors include - Banks, insurance companies, pension funds, hedge funds, mutual funds, sovereign funds etc...