A modular, open-source suite covering sales, operations, finance, HR, and marketing under one data model. You start with the apps you actually need and add more as the business grows, instead of buying a five-year roadmap up front.
Both are open-source, both avoid recurring per-seat licence fees, and we implement both. Odoo's strength is breadth, a very wide catalogue of pre-built apps that snap together. ERPNext's strength is depth in specific areas and a framework (Frappe) built for custom development on top. The right answer depends on your process, not on which one we'd rather sell you.