The STK500 is a professional development board for a variety of Atmel chips.
It's also worth noting that the STK500 is a very OLD "professional development board." It doesn't have any USB connectivity at all, for instance. In addition to the STK500, you'll need a USB/RS232 converter and a power supply before you can do anything.
Arduino includes boards that use the same sort of socket as the STK500, so on SOME arduinos you can "replace the microcontroller" about as easily as you can on the STK500. The STK500 has a BUNCH of sockets (as shown in the photo), so it supports many more types of AVR microcontroller.
The STK500 is (was) used in professional environments mostly because it is an officially supported tool from the chip manufacturer, that supports a lot of different chips, and is supported by the official tools "Atmel Studio" and "AVR Studio."
Note that the TERM "stk500" is also used to describe the communications protocol used between the stk500 board or other device programmer and your computer. The Arduino uses the same stk500 communications protocol for its "upload" function.