CATIA (Computer-Aided Three-dimensional Interactive Application) is a 3D modeling software used for computer-aided design (CAD), computer-aided manufacturing (CAM), and computer-aided engineering (CAE). CATIA V5 R21 is a specific release of the software, which is widely used in various industries such as aerospace, automotive, and industrial equipment.
Please note that downloading software via torrent may be against the terms of service of the software vendor and may also be illegal in some jurisdictions. Dassault Systèmes, the vendor of CATIA, offers various ways to purchase and download the software, including a free trial version.
If you're looking for a legitimate way to try or purchase CATIA V5 R21, I recommend visiting the official Dassault Systèmes website or contacting an authorized reseller.