A venture capitalist provides the seed money for someone else's ideas.

For example, If I have a great idea for selling pet supplies online using a cute sock puppet as a mascot, I might write up a business plan and send it to some venture capitalists (VCs).

If they agree with me that this idea's going to be successful, then they might give me $X Million to start my business in exchange for Y% of my new company.

VCs are also often involved in later stages of a startup's development, including the IPO.

The key idea is that a venture capitalist provides capital to ventures that desire $$$ to develop/expand in exchange for equity in the venture.

hth,
Bobby