I just read a post on Debashish Chakrabarty's blog titled "Primer: Difference between javac and JIT". It contains a very good explanation of what Just In Time (JIT) compilation is and how it relates to HotSpot JVMs. HotSpot JVMs identify sections of code that are worth taking the time to compile from bytecode to machine code. These sections of code are referred to as hotspots. For instance, a loop is often worth taking the time to compile to machine code rather than interpreting the byte code every time through the loop.