One thing they do is not sell to consumers anymore. For better for for worse, they got rid of that. The margins were too thing and it was harder to compete with Apple, Dell and other overseas makes of consumer harder.
They sell services, main frames, cloud, software used by enterprise (storage, project management, ...) provide consultants, and even have a bank! that companies can get financing from (when they buy all that expensive other stuff from it).
They also invented many fundamental computer hardware and software technologies. (I was a bit surprised when I first read that several years ago.) From the Wikipedia article about IBM:
IBM has 12 research laboratories worldwide, bundled into IBM Research. As of 2013 the company held the record for most patents generated by a business for 22 consecutive years.[16] Its employees have garnered five Nobel Prizes, six Turing Awards, ten National Medals of Technology, and five National Medals of Science.[17] Notable company inventions include the automated teller machine (ATM), the floppy disk, the hard disk drive, the magnetic stripe card, the relational database, the Universal Product Code (UPC), the financial swap, the Fortran programming language, SABRE airline reservation system, dynamic random-access memory (DRAM), copper wiring in semiconductors, the silicon-on-insulator (SOI) semiconductor manufacturing process, and Watson artificial intelligence.
They sell services, main frames, cloud, software used by enterprise (storage, project management, ...) provide consultants, and even have a bank! that companies can get financing from (when they buy all that expensive other stuff from it).