Aggregate planning is an operational activity that does an aggregate plan for the production process, in advance of 2 to 18 months, to give an idea to management as to what quantity of materials and other resources are to be procured and when, so that the total cost of operations of the organization is kept to the minimum over that period (Wikipedia).
Costs are minimized by adjusting production rates, labor, inventory, overtime, subcontracting and other input variables. Influencing demand through advertising or promotion may also help.
Sales and operations planning (S&OP) is an integrated business management process through which the executive/leadership team continually achieves focus, alignment and synchronization among all functions of the organization. The S&OP process includes an updated forecast that leads to a sales plan, production plan, inventory plan, customer lead time (backlog) plan, new product development plan, strategic initiative plan and resulting financial plan. Plan frequency and planning horizon depend on the specifics of the industry. Short product life cycles and high demand volatility require a tighter S&OP than steadily consumed products. Done well, the S&OP process also enables effective supply chain management.
A properly implemented S&OP process routinely reviews customer demand and supply resources and "re-plans" quantitatively across an agreed rolling horizon. The re-planning process focuses on changes from the previously agreed sales and operations plan, while it helps the management team to understand how the company achieved its current level of performance, its primary focus is on future actions and anticipated results (Wikipedia).
APICS defines S&OP as the "function of setting the overall level of manufacturing output (production plan) and other activities to best satisfy the current planned levels of sales (sales plan and/or forecasts), while meeting general business objectives of profitability, productivity, competitive customer lead times, etc., as expressed in the overall business plan.
A chase aggregate plan means that the amount of input (resources) are matched in each time period with the required output. For a chase plan the pros are: no wasted resources with over-production or lost revenue with under-production. The cons are: labor and other resources are constantly being increased and decreased, leading to additional costs of hiring, firing, training and potential poor quality.
A level production plan means that production is scheduled and resources are acquired to produce product at a constant level over the planning period. For a 12 month level production plan, the annual demand is forecast, and then 1/12th of the demand is scheduled for production each month. For a level plan the pros are that production is constant, leading to higher productivity, better quality and a better work environment due to stability. The pros are that when production is higher than demand due to demand fluxuations, inventory is built up (costing $). When production is lower than demand, if there is not adequate inventory, customers can experience shortages and sales may be lost.