Aggregation operations process data records and return computed results. Aggregation operations group values from multiple documents together and can perform a variety of operations on the grouped data to return a single result.

MongoDB provides three ways to perform aggregation:

1. Aggregation pipeline
2. Map-reduce function
3. Single-purpose aggregation methods

db.orders.aggregate([
   { $match: { status: "A" } },
   { $group: { _id: "$cust_id", total: { $sum: "$amount" } } }
])