The limited partners provide the capital but do not take part in managing the fund or advising the portfolio companies, which is a major responsibility of their counterparts (the general partners).
Like the shareholders in a corporation, limited partners also have limited liability, that is, liable only to the extent of their original investment, and are protected in general from any further losses and legal actions.
The general partners manage the funds and assume the financial and legal obligations. The limited partners have priority over the general partners upon liquidation and receive a large fraction of the capital gains proportional to their original investments as defined by the partnership agreement.
They can be wealthy individuals or corporations and can choose to invest in limited partnerships instead of directly in the companies because they do not have the expertise in the field nor access to information in the private equity market that the general partners have. They pay the general partners a fee to cover the costs of fund management.