GOMS (which stands for Goals, Operators, Methods, Selection Rules) is a technique to break down a user task into its individual steps. In the GOMS model, goals are achieved by completing a set of operations (method). There can be multiple methods available to they user of which they choose one based on the selection rules. For more complex tasks, goals are allowed to be divided into several subgoals.
(The concepts behind a GOMS model and their relationships, Olli Savolainen, licensed CC-BY-SA 3.0)
GOMS are very useful for analyzing the complexity of a task and the potential hurdles users have to face. However, GOMS does not account for errors and inexperience, making it only applicable to the average user. Another drawback of "standard" GOMS (called CMN-GOMS) is that multitasking is not consid