For any enterprise that needs to implement an ERP system, the basic modules that are essential would include all the major functions of the enterprise that describe it completely.
OpenERP is one such system that does it all and has all the modules to cater such requirements and even more, but talking about main modules, these modules are always required and should always be installed. Some of the major and very important modules are:
account: this module is responsible for accounting and financial management. It covers:
Cost / Analytic accounting
Third party accounting
Customer and Supplier Invoices
Reconciliation process by partner
Other modules like account_accountant manages all the journal entries, account_voucher manages payments and depend on account module.
sale: this module manages all the sales related tasks. It covers:
Quotations to Sales Order
Sales Order to Invoice
Invoicing methods: invoice on orders, delivery, time-sheets, and advance invoice
Products and prices
point_of_sale: manages a quick sale process. It covers:
Allow payment mode and split one payment mode into multiple
Compute money to return
Create and confirm picking lists
Create quick invoices and refunds.
stock: this module manages all the inventory and inventory related functions, can even manage multi-warehouses and multi-locations. It covers:
Moves history and planning
Stock valuation like standard or average price
Automatic reordering rules as stock level, JIT
Bar code support
Detection of mistakes through double entry system
Traceability as upstream/downstream, production lots, serial number
purchase: this module manages all the purchases for goods from the supplier.
mrp: this module manages the manufacturing process. It covers:
Make to Stock / Make to Order (by line)
Scheduler computation periodically / Just In Time module
Different reordering policies
Costing method by product: standard price, average price
crm: this module manages customer relationship. This module manages:
leads, opportunities, meeting, phonecall etc
communication, identification, prioritization,
assignment, resolution and notification.
ensures cases successfully tracked by users, customers and
send reminders, process the requests automatically
hr: this module manages all the human resources. This module manages employees and employee hierarchy, the hr department, and hr jobs. Though we have other modules that depend on this module like hr_attendance, hr_payroll, hr_holiday, hr_payroll_account, hr_recruitment, hr_evaluation. These can be installed as per your requirement.