ZIP and RAR are both archive formats that compress files into a single package. They've coexisted for decades, and each has its strengths.
ZIP wins on universality. Every operating system opens ZIP files natively — no software installation required. The format is open and royalty-free, which is why it's the default for email attachments, software distribution, and web downloads. ZIP compression is fast and good enough for most purposes.
RAR wins on compression ratio. RAR files are typically 5-15% smaller than ZIP for the same content, thanks to more aggressive compression algorithms and solid archive mode (which compresses files together rather than individually). RAR also handles large archives better, with built-in support for recovery records (to repair damaged archives) and multi-part splitting.
The catch with RAR: you need third-party software. WinRAR (Windows, paid with infinite "trial") is the standard tool. 7-Zip (free) opens RAR files but can't create them. On Mac, The Unarchiver (free) handles RAR. No operating system opens RAR natively.
For most people, ZIP is the right choice. Use it for sharing files, email attachments, and general-purpose archiving. RAR is worth using when you need maximum compression for large archives and the recipient has software to open it. For the best compression overall, consider 7z format — it beats both ZIP and RAR but has even less native support.