In its simplest form, planning is nothing more than sequencing and elaboration; that is, deciding what order to get things done, and then determining a more detailed manifest of work items required to produce each deliverable.
Sequencing: determining the desired order of delivery.
Elaboration: determining a detailed manifest of work items to produce a deliverable.
While I say this and hold it to be true, many will argue that planning is much more than this. To those, let me say this only: If there were no external constraints on the work – and you were the sole stakeholder (like Bill Gates doing a household project) – it would just be sequencing and elaboration. Every other aspect of planning is optimization against external constraints.
I have been saying for several years now that sequencing is a simplification of scheduling. By this I mean that, when I don't have enough information to produce a reliable schedule, I start with a sequence. I do this, so that I can start working to generate the information to produce the schedule. This method works all the time at the beginning of a complex project where people get paralyzed because they don't know where to start.
Today, I flipped this over. Sequencing is not a simplification of scheduling – scheduling is an optimization of the plan to meet an external time constraint. When I think of this, what I realize is that planning starts with a rational sequence of value delivery, and then becomes complex as we optimize to comply with external constraints – time, budget, resource availability.