The Intel 80386 (sometimes 386 or i386 for short) is the 4th generation microprocessor CPU from Intel based on the 8088/8086 CPU. The 386 was a 32-bit CPU, featuring all the features of the Intel 80286 CPU, plus 32-bit protected mode with variable page sizes, allowing for a flat memory space where the entire 4GB of accessible RAM could be accessed without segmentation. The 386 was introduced in 1985, and finally discontinued in 2007.
The 386 also featured a special mode called v86 which facilitated the creation of VDMs. In this mode all un-privileged instructions of the 386 could execute in a hardware virtual machine, and privileged instructions would fault.
The most popular reference is the INTEL 80386 PROGRAMMER'S REFERENCE MANUAL.
The 80386SX was a low cost version of the 386. Instead of having a 32-bit address bus, it was restricted to 24 bits, meaning the 386SX could only address 16MB of RAM maximum. The 386SX also only could transfer data 16 bits at a time, so reading a 32-bit word took two reads. This basically kept the 386SX comparable in speed to the 286. The internal registers were still 32bit wide, so it could run the same software.
The first 386 CPUs had an issue with the multiply instruction in 32-bit modes, so there had to be a recall on all parts. Later the parts were either stamped "16 bit software only" or with a double sigma sign to certify they would operate correctly.
IBM was not the first PC manufacturer to make a 386 CPU, they were too infatuated with the 286 and the 16-bit version of OS/2 so it was Compaq who beat them to the first 386-based machine, with the Desqpro 386.