Extreme macro is macro photography with a ratio bigger than 1:1.
For example by using extension tubes between your camera and macro objective you can achieve a magnification bigger than 1:1.
Also conversion objectives can be used to achive this, like for example the Raynox DCR250.
Me personally, I like to work with microscope objectives. With an RMS adapter I can place it on my Nikkor 70-200mm 2.8.
There are 2 kinds of microscope objectives: Infinite and Finite objectives. The biggest difference between the two is that with Infinite objectives, another objective has to be placed between your camera and the microscopic objective. With Finite objectives you can place extension tubes or a balow in between your camera and the microscopic objective.
Because these methodes result in a very shallow depth of field, often a macro rail is used. With the macro rail you take multiple photo’s with a changing focus point. And later you will combine al these pictures into 1 sharp endresult. This is called Focus Stacking