The difference between Tier 1 and Tier 2 storage used to be intuitively obvious. Tier 1 storage arrays came packaged in a refrigerator-like box, while Tier 2 storage arrays fit neatly in 19-in. racks. While still not a bad way to ascertain the difference between storage arrays, that justification may sound a little hokey on a business case.

Since it seems (from this side of the fence at least), that no two vendors use the same terminology - unless they mean something completely different by it, it's very handy to get summaries like this from vendor-independent experts...