Pearlite is a microstructure occuring in Steel. It consists of alternating layers of Iron in Body Centered Cubic structure (Ferrite), and Fe3C (Cementite).
It is called Pearlite because of the fact that when viewed under low magnification, it looks a bit like mother of pearl.
Compared to other Steel microstructures, specifically Bainite and Martensite, it's not too strong or hard. However, it is quite ductile, which often comes in handy if you wish to avoid sudden catastrophic failure of whatever it is you're building.
How do you make Pearlite? Well, you take some Austenite, basically any steel above 727C, and let it cool. Pearlite will only be formed if it's allowed to cool between 540C and 727C. At the lower temperature, it forms quite quickly, within 10 seconds. At the higher temperatures, it can take quite a bit longer, taking longer the closer it is to 727C.
There are generally two classifications of Pearlite, Fine Pearlite, and Coarse Pearlite. The physical difference between the two is in the thickness of the layers the Pearlite is made of.
The finer the layers making up the Pearlite, the more they're going to resist slipping relative to each other, and the stronger the Steel is going to be. Conversely, the more coarse the Pearlite, the more the layers will be able to stretch out, and the more ductile it'll be.
Finer Pearlite is formed at lower temperatures than Coarser Pearlite, and thus it has to be cooled down quicker than the Coarse. Generally, if you wish to form Fine Pearlite, simply pulling it out of the furnace and letting it cool in the air will be sufficient. To form Coarse Pearlite you have to stick it in another furnace, one that is kept between 600-700C.
Also, Pearlite will generally not form on its own, depending on the Carbon content of the Steel. If the Carbon content of the Steel is less than 0.76%, some of the Austenite will have already transformed into Ferrite before it reaches 727C, and if it's above 0.76% Carbon, some of it will have turned into Cementite.
Usually these will form branches that will clump together a bit, and then the Pearlite will fill the gaps between them. This is called Pearlite in a Ferrite (or Cementite) matrix.
Of course, if you've got exactly 0.76% Carbon, you can get pure Pearlite.
The less Carbon you have, the more Ferrite you'll have compared to Pearlite, and the more Carbon you have, the more Cementite you'll have crowding out the Pearlite, up until 6.7% Carbon, at which point it'll be all Cementite.
And, if you really want to get funky, you can cool it down below 540C while the Pearlite is still only half done forming. If you do this, the Austenite that hasn't turned into Pearlite yet will turn into either Bainite or Martensite, depending on how fast you cool it.
This is how you get some funky properties, but it's beyond the scope of this node.
Sources:
William D. Callister, Jr., Materials Science and Engineering An Introduction, 5th Edition. John Wiley & Sons, Inc.
My memory from ENME421,
Mechanical Engineering, Materials I, at the
University of Calgary.