This is a brief, but hopefully illuminating, explanation of decoherence. It is based on a presentation I gave at the AAPT (American Association of Physics Teachers) meeting in summer 2010. This online version adds a section about two-slit interference.
Here's the idea. The Schrodinger's cat paradox asks why we don't see superpositions of macroscopic objects like cats. The answer is that any such superpositions will experience decoherence long before we can observe them. Decoherence turns a superposition into a pure probability. This presentation first explains superpositions, then interactions using multi-particle wave functions, and then decoherence itself. The section on two-slit interference shows how measuring which slit the particle goes through has a very similar effect to decoherence.
Here are the links:
Scott Johnson,   2010