That percentage doesn't usually include the lawyer's costs and expenses. Be sure to ask whether your lawyer collects the percentage before or after other costs have been deducted. It can make a big difference. Most lawyers want the fee paid before expenses but it is negotiable. You can negotiate a lower rate if the case settles quickly and doesn't go to trial.
Some lawyers charge a fixed fee for routine legal work.
Retainer fees are basically down payments on some future bill. The retainer is a specific amount paid regularly to guarantee the lawyer will be available to help when you need it and provide consultation. Businesses and corporations generally use the retainer. Unless you have lots of legal work over an extended period don't expect to use a retainer. Hourly fees vary widely depending on the expertise of your lawyer, the complexity of the legal specialty, whether he or she is from a large firm or from an urban area.