An attorney has to keep the property of a client or third party separate from the attorney's own property. That means client funds must be kept in a trust account compliant with your state bar's requirements. Additionally, the attorney must keep the client informed, and maintain records of client property, and deliver funds or property to the client or a third party if they are entitled to those items. (Look out: I've seen more than one state bar essay fact pattern where an attorney delivers money or property to a client well past the initial time that the client was entitle to receive it.)
Another common scenario to remember: If a lawyer is in possession of money or property, and the client disputes the lawyer's claim to the money/property, the lawyer must keep separate the disputed amount/property (usually in a trust account).