A venture capital firm is an organization that pools funds to invest in high-risk / high-growth companies.
Limited Partners (also known as LPs) provide funds to an individual venture capital fund. That fund is invested by General Partners (also knows as GPs) of the venture capital firm into companies, such as early-stage technology startups. When those companies go on have an initial public offering (or IPO) or get acquired, the profits from those investments are then shared between the LPs and GPs.
A venture capital firm manages one or more venture capital funds. An individual fund is typically designed to exist for 10 years, but only the first two to four years are for initial investments. Thus, most venture capital firms raise new funds every two to four years in order to continually be able to make new investments into companies.