If you're referring to the 211 and 211/k, then they're both single satellite tuner receivers wiht a single OTA tuner built-in. The 211k is smaller and black.
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Just about no difference between the two. They'll get you the same channels, but as mentioned, the 211k is black, and much smaller than the 211 (regular 211 is silver).
211k's have an embedded smart card (chip), 211's can have external or internal smart cards. As Iceberg mentioned, when you check the receiver and SC numbers, the newest generation of SC's (G3) will start with S19
If the receiver is not sold with a card, how do I find the S#?
If you're able to hook it up, press menu twice and it will have the smart card number listed in the system info screen. If you're buying it online, ask for the receiver and smart card numbers. If the receiver is coming from us, it will have a new SC, if it does not have a new SC, one can be ordered.