There are a few different situations when getting hired through another company, which makes it important that you read the ads correctly, and if needed ask questions.
This is important to look for:
- Are you recruited by the recruiting-company to be hired by the company where you are going to work,
- are you hired by the recruiting-company to be placed at that other company?
If it's the first, the company has only outsourced the recruiting process to that other company as recruiting people is expensive (and in some fields a recruitment process may end up fruitless) but hiring someone else to do it for you can be cheaper.
If it's the later, you may want to think twice about it. When you are recruited by a man-power company you will earn less money than if you had been hired by the client directly. The man-power company charges the client (the company where you will be working) the wage you should be getting, but they cut a certain percentage off of that money for themselves, leaving you a smaller wage.
My advice is to cut out the middle man, if you can. Sometimes working for a man-power company can be beneficial though as it can serve as an introduction to workplaces where you can lobby for them to hire you directly; sometimes hiring you can be beneficial for the client company and yourself.