Let's start at the beginning:
The Astra group of satellites (which your dish is pointing at) broadcast lots of FTA (= 'free to air') channels, which can be received by any generic satellite receiver. (You can use a Sky box, with the old card in it, without a subscription to function as such a receiver). Those channels are listed here:
http://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
Sky can also sell you a 'Freesat from Sky' card (which, confusingly, is not the same as 'Freesat') for a one-off payment (i.e.with no subscription). That basically does little more than an old (ex-subscription) Sky card would but possibly adds in things like a few 'red button' options.
'Freesat' receivers are designed to receive a
subset of the channels in the list (from my link above), together with a few 'red button' extra facilities. (i.e. Freesat actually gives you fewer channels than a generic satellite receiver but you'd probably not miss things like obscure religious stations anyway!).
All (basic) Freesat boxes can receive the same channels. They also provide 'catch up' facilities through BBC iPlayer and ITV Player (except in Scotland or Northern Ireland, where there's only the BBC iPlayer available). However there are now also 'Freetime' boxes available; they have the same channels as ordinary 'Freesat' boxes but additional 'on demand' facilities (such as 4oD and Demand 5). It's all explained here:
http://www.freesat.co.uk/channels
UKTV, which owns (among other channels) Dave, Drama, Really and Yesterday do NOT make those channels available as 'free to air' on the Astra satellites. They're encrypted and the ONLY way you can receive them through a satellite dish is with a Sky subscription. However they're available free of charge on terrestrial TV (i.e. Freeview).
Challenge, Pick and CBS Reality are all in my first link, so they're available for free using any satellite receiver. They're also available on (terrestrial) Freeview.