Dmidecode reports information about the system's hardware as described in the system BIOS according to the SMBIOS/DMI standard.

This information typically includes system manufacturer, model name, serial number, BIOS version, asset tag as well as a lot of other details of varying level of interest and reliability depending on the manufacturer. This will often include usage status for the CPU sockets, expansion slots (e.g. AGP, PCI, ISA) and memory module slots, and the list of I/O ports (e.g. serial, parallel, USB).

Beware that DMI data have proven to be too unreliable to be blindly trusted. Dmidecode does not scan the hardware, it only reports what the BIOS told it to.