Introduced in 1989 as successor to the 386. It included a floating point unit (FPU) (except for 486SX, which came later) and contained more than one million transistors.
The instruction set is very similar to the 386 as it only has a few extra instructions. Performance-wise the 486 is a big improvement. It has an on-chip unified instruction, data cache, built-in FPU and enhanced bus interface unit.
Other differences between the 386 and 486 can be found in on-chip SRAM cache (8KB in total, 16KB on later models). The 386 did not have on-chip cache, but could be paired up with off-chip cache.
Intel released the 486 and made models up to 100MHz. Other companies also made 486 CPU's by enhancing the Intel 386 design (like AMD) or by reverse engineering (Cyrix). Some of the 'copies' performed less, others equal and some out-performed the fastest Intel 486.