Typically, with any game that is viewed from a competitive standpoint, there are strategies which rise to the top over time. The very best strategies are considered "top-tier". Every other strategy, besides these top-tiers, are organized by how close they are to optimal. Most of time, these strategies will be clumped into "tiers" of competitiveness.
To answer your second question, most commanders are multi-colored for two main reasons. For one, the mana base in competitive EDH is so good that there are typically very few penalties to running an additional color. Secondly, there are colors which have very powerful format staples. Black has vampiric tutor and necropotence. Green has very good mana dorks. Blue has cantrips and countermagic. No matter what color you intend to run, your CEDH deck WILL be improved by adding one or more of these colors. On top of all of this, the partner commanders are especially good by their very nature and allow you access to four colors.