To be a partner means that you go from being an employee of the firm (and being paid a salary) to becoming a part-owner of the firm and sharing in the firm's profits (and liabilities). However, this is not always the case. Because of changes in the law industry, what it means to be a partner has changed.
The majority of those who achieve the position of partner with their firm become non-equity partners. This is a relatively new concept in the legal field.
There are different tiers of partnership in most law firms now, which means less job security for non-equity partners. As a partner, they are still expected to bring in a certain amount of business to the firm just like non-equity partners are expected to. If the non-equity partners do not bring in enough new business, they can be dismissed from the firm.
Non-equity partners are essentially middle management and equity-partners-in-training. They get a bigger salary than regular lawyers and a nicer-sounding title, but in most other ways are not any different than non-partner lawyers.
On the other hand, equity partners do have job security. It means you are performing consistently at the level your law firm expects, and as long as you continue to do so, your job is secure.
Once someone is made an equity partner, they are given a loan to "buy-in" to the firm. This means they become a part-owner and get part of the firm's profits in addition to their salary. The cost to "buy-in" is usually in the tens of thousands of dollars.
Those who can afford to pay it on their own can do so without getting a loan from the firm. Equity partners, in this light, are more invested in the success of the firm because they have a financial stake in it. When the firm does well financially, they do well financially and vice versa.