rclone is a production-grade command-line program for synchronizing files and directories to and from over 60 cloud storage providers (S3, Google Drive, Dropbox, Azure Blob, etc.) and local filesystems. It implements a clean, modular architecture centered on the `fs.Fs` and `fs.Object` interfaces, which all backends implement. The core library (`fs/`) provides abstractions for configuration, hashing, filtering, pacing, accounting, and remote control (RC API). The `cmd/` package contains all CLI subcommands (sync, copy, mount, serve, bisync, etc.), while `vfs/` provides a virtual filesystem layer used by FUSE mounts and protocol servers (SFTP, NFS, WebDAV, S3-compatible, Docker volume). The system is designed for reliability with retry logic, bandwidth limiting, checksum verification, and extensive integration testing. Key entry points include `rclone.go` (main), the Cobra command tree in `cmd/`, and the RC HTTP server (`fs/rc`). Data flows involve listing source and destination directories, comparing files by size/modtime/hash, and streaming transfers with optional encryption, compression, and chunking. External dependencies include cloud SDKs (AWS, Google, Azure, etc.), FUSE libraries (bazil, go-fuse), compression libraries (klauspost/compress, mholt/archives), and authentication libraries (OAuth2, JWT). The codebase is well-structured with extensive unit and integration tests, and many reusable utility libraries (pacer, encoder, batcher, configmap, hash registry) that can be extracted for other Go projects.