A retainer agreement is a work for hirecontract. It falls between a one-time contract and full-time employment. Its distinguishing feature is that the employer pays in advance for work to be specified later. Additional contracts regarding the performance of this work may also apply.
It is common for a person seeking the services of a lawyer (attorney) to pay a retainer ("retainer fee") to the lawyer, to see a case through to its conclusion. A retainer can be a single advance payment or a recurring (e.g. monthly) payment.
A retainer fee can be paid on a fixed, pre-negotiated rate or on a variable hourly rate depending on the nature of retainer and also, the practice of the lawyer/advocate being retained. Both models exist in the industry. The purpose of a retainer fee is to ensure payment for future services or work to be rendered. Absent an agreement to the contrary, a retainer fee is refundable if the work is not performed.
Retainer is the basis of authority for an advocate. It limits the authority because it is not for all general purposes for all time. It is therefore specific in nature e.g. during litigation the advocate is authorized by the client to accept service of proceedings that do not require personal service on behalf of the client. This kind of authority can either be expressed, implied, apparent or usual through the usual practice of the advocate while pursuing the instructions of the client.