A retainer agreement is a work-for-hire contract. It falls between a one-off contract and permanent employment, which may be full-time or part-time. Its distinguishing feature is that the client or customer pays in advance for professional work to be specified later. The purpose of a retainer fee is to ensure payment for future services or work to be rendered.
Time and materials (aka T&M) is a standard phrase in a contract for construction, product development or any other piece of work in which the employer agrees to pay the contractor based upon the time spent by the contractor's employees and subcontractors employees to perform the work, and for materials used in the construction (plus the contractor's mark up on the materials used), no matter how much work is required to complete construction. Time and Materials is generally used in projects in which it is not possible to accurately estimate the size of the project, or when it is expected that the project requirements would most likely change.
At Gopler, we use different tools to track retainer and time & materials arrangement, such as Jira, Redmine, dotProject, Mantis and Bugzilla. We are also flexible to use system that is recommended by the client.