Glurch is very similar in behavior to silly putty (meaning it is also a non-Newtonian fluid). You combine Elmer's school glue and water in 50-50 ratio. Try and make the mixture completely homogeneous - in other words, no areas of extra water or glue, blend them completely. Then make a saturated solution of water and Borax. Borax is a laundry booster which helps lift stains. You will probably need a teaspoon for every cup of water. Add a small amount of this solution to the glue/water mixture and stir until it gloms together. It looks pretty slimy at first, but as you play with it more the extra water is removed and the Glurch becomes more putty-like.
Glue contains a polymer called polyvinyl acetate. A polymer is a very long chain of monomers (the basic molecule which is the building block of the polymer). These chains are all independent, floating around and tangling up. Borax contains sodium borate (Na2B4O7 • 10H2O). The sodium borate becomes boric acid (H3BO3) when dissolved in water. The boric acid causes cross-linking in the polyvinyl acetate. Cross-linking is a covalent bond between the chains. So if before the polymer was like cooked spaghetti, now it's mutant pasta, with all the strands stuck together in different places.
The cross-linking is what makes the Glurch less like a liquid. The polymers can't move as freely anymore. The more borax you add, the more cross-linking occurs.
In addition to being classified as a non-Newtonian fluid, Glurch is also a viscoelastic material. According to the dictionary, this is a substance that "exhibits both viscous and elastic characteristics when undergoing deformation". One interesting property of viscoelastics is that they don't return to their original form when stretched for a long time. In something elastic (like a Slinky), no matter how many times you stretch it, it will always return to its original shape (unless you stretch it so hard it reaches the plastic range). Viscoelastics don't. For example, a rubber band stretched around a newspaper applies less force over time as it deforms. This is called stress relaxation.
If you want to learn more about viscoelastics, try this page. While it is geared toward teachers, it's the best general overview I've found so far.
Bonus activity: After you've made the Glurch, blend in some ferric oxide and it will become magnetic!