WinRAR is able to create archives in two different formats: RAR and ZIP. Here is a comparison of these formats:
ZIP archives
The main advantage of ZIP format is its popularity. For example, most archives on Internet are ZIP archives. So if you are going to send an archive to somebody, but you are not sure that your addressee has WinRAR to extract the archive contents, it might be a good idea to use the ZIP format. Though, you may also send a self-extracting archive. Such archives are slightly larger, but can be extracted without external programs.
Another ZIP advantage is speed. ZIP archives are usually created faster than RAR.
RAR archives
Typically RAR format provides better compression than ZIP, especially in the solid mode. WinRAR multi-volume archives in RAR format allow more functions like self-extracting volumes.
RAR format has also a few important features missing in ZIP, such as the recovery record allowing to reconstruct damaged data and locking important archives to prevent their accidental modification.
Both RAR and ZIP formats are able to handle files of practically unlimited size (up to 8,589,934,591 GB).