Better Auth is a framework-agnostic JavaScript authentication library that runs inside your application and stores user data in your own database. It provides built-in features like email/password authentication, social SSO, 2FA, passkeys, and multi-session support through a plugin system. Unlike SaaS solutions, it eliminates per-user costs and vendor lock-in while offering full control over authentication flows. The library works with popular frameworks including Next.js, Remix, Nuxt, and SvelteKit, providing a unified authentication approach across different tech stacks.