A bill of quantities (BOQ) breaks a project into measurable items such as excavation, PCC, reinforcement, brickwork and plastering. Each item has a description and specification, a unit (m³, m², kg, nos), an estimated quantity taken off the drawings, and a rate. Quantity times rate gives the item amount.
Clients use the BOQ to invite comparable bids. Contractors use it to price the job, then as the backbone of the project: the budget is built on it, progress is measured against its items, and every RA bill claims quantities item by item.
Work that is not in the BOQ, or quantities that change a lot, are handled as a variation or extra item. Giving each item a cost code lets you compare budget vs actual later. A BOQ with vague item descriptions causes arguments at billing time, so specify materials, finishes and what each rate includes.
Example
Illustrative item: "Brickwork in CM 1:6, superstructure", unit m³, quantity 250, rate ₹6,000, amount ₹15,00,000.
BOQ & budgeting in CivilPilot
BOQ, approved budgets and budget against committed and actual spend for every cost code.

