What is an incubator, and are they useful? Are there risks?

I've seen a lot of people briefly mentioning incubators in the contexts of start ups here. I can surmise that they are designed to help smaller businesses get through the early stages in some way, but I'd like to know more about them.

What do they do exactly? What benefits (if any) do they typically offer, and are there any downsides? Do I risk losing my own intellectual property or money if I work under another organisation?

An incubator (or accelerator), especially in the start-up scene, selects businesses to come in for a defined period of time, in which they are normally given free space to work and sometimes seed funding in exchange for equity.

Incubators can have a large network and can introduce you to mentors and investors to help you grow further. They normally have many mentors on hand to help guide your business and provide networking opportunities. The IP developed under an incubator normally belongs all to your company, but as mentioned the incubator may require equity in your company to be accepted in.

Essentially you lose equity for experience, networking and cash.

The only knowledge I have of incubators is based on Startup Edmonton. As far as I know, they do not take any equity from the company (please correct me if I am wrong!) but instead charge you a small fee for membership. They give you access to workspace, workshops and mentors that will help you get your business off the ground, as well as an environment that is incredibly energetic and motivating.

This may vary by incubator though. I must admit that I have no knowledge of how any other incubators work.

If you are a student, you should look into what opportunities your University/College offers. Many Universities offer students programs that are very similar to incubators but do not charge as large of fees or require you to part with any equity. For example, the University of Alberta (Canada) is partnered with Startup Edmonton to provide free memberships to students; this means that there is no downside to becoming a member.