Open source software runs underneath nearly every commercial product, which means nearly every company is bound by license terms no one on the deal team ever read. This guide explains what "open source" means as a precise legal term under the Open Source Initiative's ten-criterion definition, walks the license families from permissive MIT, BSD, and Apache through weak-copyleft LGPL and MPL to the reciprocal GPL and network-reaching AGPL, and works through the field's two hardest questions: when a code combination becomes a "derivative work" that triggers copyleft, and when two open licenses may lawfully be combined at all. It traces how volunteer-drafted licenses became enforceable law through Jacobsen v. Katzer, Wallace v. IBM, Artifex v. Hancom, SFC v. Vizio, the BusyBox suits, and Neo4j v. PureThink, and it lays out the modern compliance toolkit built on the software bill of materials. It then maps the live 2026 controversies: the relicensing wave from MongoDB's SSPL to HashiCorp's BUSL, the open-core model, and the collision of open source with generative AI. The aim is a single reference a general counsel, an engineer, a judge, and a curious non-lawyer can each follow end to end.