Tracking stock is not just about adding and subtracting quantities. Some complications arise when:

Back-dated (past) entries are made / edited: This affects future stock levels and may lead to negative stock.
Stock has to be valued based on First-in-First-out: ERPNext needs to maintain a sequence of all transactions to know the exact value of your Items.
Stock reports are required at any point in time in the past: You have to lookup what was the quantity / value your stock of Item X on date Y.

Selling is the communication that happens with the customer prior to and during the sale. You might be managing all the communication yourself or you may have a small team of sales people to handle this. We help you track the communication leading up to the sale, by keeping all your documents in an organized and searchable manner.

If your business involves physical goods, buying is one of your core business activity. Your suppliers are as important as your customers and they must be provided with as much accurate information as possible.

Buying in right amounts, in right quantities, can affect your cash flow and profitability.

ERPNext helps you to maintain multi-level Bill of Materials (BOMs) for your Items. It helps in product costing, production planning, creating production orders for your manufacturing shop floors and planning inventory by getting your material requirement via BOMs (also called Material Requirements Planning MRP).

ERPNext helps you manage your Projects by breaking them into Tasks and allocating them to different people.
Purchasing and selling can also be tracked against Projects and this can help the company keep tabs on its budget, delivery and profitability for a Project.
Projects can be used to manage internal projects, manufacturing jobs or service jobs. For service jobs, Time Sheets can also be created that can be used to bill Customers if billing is done on a Time & Money basis.

The Human Resources (HR) Module covers the processes linked to managing a team of co-workers. Most important feature here is processing the payroll by using Process Payroll to generate Salary Slips. Most countries have complex tax rules stating which expenses the company can make on behalf of the Employees. There are a set of rules for the company to deduct taxes and social security from employee payroll. ERPNext allows to accomodate all types of taxes and their calculation.

ERPNext helps you track business Opportunities from Leads and Customers, send them Quotations and make confirmed Sales Orders.

The CRM Module helps maintain Leads, Opportunities and Customers.