Improvements include greater stability, "plenty" of bug fixes, greater versatility and additional interface-options.
Microsoft describes the release as "much lighter on its feet, with plenty of bug fixes and impressive new stability and versatility."
There's the choice of two file formats for download – .bin or a .hqx. The .bin file downloads faster but is less compatible with older Macs.
Internet Explorer 5.1 is available as a free 5.4MB download.