DTH means Direct to home services of cable tv thru Set-top-box & a small dish antena install on the roof & other service is CAS (Conditional Access system) which is provided by your local cable operator.This is also provide thru digital Set-Top-Box, In the CAS you have option to choose the pay channel & Govt. (TRAI) has fixed the rate of channel that is Rs.5/-per pay channel & can also choose a readymade packages of channels & in DTH service there is no option of choose individualiy channel, Both have digital quality, DTH intial cost little expensive then CAS, DTH service may down in rainy, cloudy, fogy, stormy season because of KU Band Technololgy which is not completey successful in india on the other hand, CAS is totally weather proof because it has C-Band Technology which is already succeed in India, CAS has more channels in comparison to DTH, Compared to both i think CAS is more better than DTH .
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Several answers have already addressed the question. Some additional information:
Similarities with CAS - you need a set top box for each TV! There are packages in both CAS and DTH, where you take multiple connections in the same home, but pay less - for example only 50% for each additional connection etc.
Choice of channels: Not all channels can be individually chosen. In the case of TataSKY, there are only 4 packages (it was worse - there was only one common package earlier!). In Dish TV, there are basic and add-on packages / channels. But you at least have transparency. In CAS, it really depends on your local operator.