In general, a charity is any organization that raises money for the benefit of those in need. It's a very broad category.

However, charitable donations are only treated specially if the charity complies with organizational requirements set out in your local charitable organizations laws. For example, in the US, charitable donations are tax-deductible if the charity is organized under s. 501(c) of the tax code, which sets out requirements as to organizational structure, charitable purpose, and a number of other conditions which determine whether an organization's charitable purpose justifies special treatment.

There are generally limits on executive compensation to try to prevent the most obvious diversions of charitable money into private pockets, and universally charitable organizations are prohibited from paying dividends to their owners or board members. Many jurisdictions require registered charitable organizations to file public financial reports regularly, which gives the public information they can use to decide whether a charity makes efficient use of their donations or not, as well. There are generally serious consequences for mishandling this, up to and including criminal charges and the loss of charitable status.

There are plenty of very inefficient charities out there, unfortunately. Effectively-run charities often spend upwards of 90% of donations on their charitable mission, but there are some out there who spend less than half the money they receive on their mission, spending the rest on operating costs and staff. Whether that's acceptable to you is a personal decision as a donor; it's worth looking into the financials of a charity before you donate large amounts of money.