non-HD digital boxes get the standard definition digital channels. What you get depends on your cableco, but it's generally additional channels, plus the digital simulcasts of the analogs.
I had one for a few years before getting an HD box, and got the extended basic from TWC, and their "variety" tier which had things like BBC America, Sleuth, Speed, and a few things like that. Your cableco's website should say what packages their digital tiers provide.
Digital refers to the delivery to the box itself. Digital transmission is more bandwidth friendly than analog, which is why the cablecos like it (more channels = more variety for customers = more customers.) The box will convert it to analog for standard def TVs, as I don't think many of them if any had digital inputs.
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