I use a lot of terms in my writing. I figured I would explain a few of them today, since someone asked nicely.
Agent is a member of the Indirect Channel. Also called a channel partner. An agent is an independent sales person for the carrier or vendor. Agents get paid a commission for closed sales. Agents may or may not work through a Master Agent. A Master Agent gets a big contract with a carrier, so that other agents can sell off it and get paid. Today, Master Agencies need to be more like the back-office partner for independent agents, so that the indie agent can just sell and stay in front of customers, without having to jump through a thousand hoops to place a single MIS order. (Do you hear me Ma Bell?)
VAR's are value-added resellers. Originally, this meant folks who built computer systems. Now, it means the company that you buy your computer and network hardware from, who also do maintenance, installation, and other value-added services, like back-up, hosting and more.
Managed Services are anything that you outsource like computer maintenance, router monitoring, IDS/firewall monitoring. MSP World at ITEXPO in October in LA will showcase some of the most progressive MSP's in North America. This show is in run by MSPAlliance. (MSPMentor writes about everything MSP. The VAR Guy may say VAR, but the blog leans towards Managed Services, probably because they are both owned by the same media company).
SI's are systems integrators. In the days when Novell was tops (and believe me nothing could stand up to Novell 3.12 server), SI's were the guys that would take various databases and systems and tie it all together. So if you were migrating from IBM DB2 to MySQL and adding web capability, you would get an SI. I guess today, they are web dev folks or consultants or something else because I haven't seen anyone call themselves an SI in a while.
Tech Data, Ingram Micro, CDW, ScanSource, NETX are distributors of hardware. You want Cisco gear, you can't get it directly from Cisco. You have to buy it through a distributor. The distributor is the one-stop shop for the build-out: phones, switches, cables, widgets, router, IAD, licenses, etc. VARs and MSP's (and MCP's) all buy from distributors.