Tier 2 might have Dell Powerconnects and HP Procurves and Cisco 2000 series products. These are good stable well performing products and are gobbled up in heaps by small and medium businesses. These are the usual choice for small enterprises and come in managed and unmanaged, layer 2 of layer 3, power over Ethernet of not or a combination of those.
You can add D-Link to Tier 2. Managed, some come with PoE ports, yada yada. Heard of leaky HP switches? Double dealing D-Links? A long time ago, yes.
Tier 3 contain your Linksys, DLink and Zyxel brand products. They basically just get the job done, but might need reset every now and then and probably can't run more then 2 ports at a full 1GBe simultaneously. They are for home use and are prices as such. Some will be better then others and some might be very good, but they are not designed for business use and thus shouldn't be used as such.
You missed Surecom.