If two airlines codeshare on a flight, you can think of it a bit like the two airlines each flying half an aircraft on the route. They will market it and sell it separately. In the purest bare codeshare:-
(a) the two airlines will still be required to compete against each other even though they are selling seats on the same aircraft - which is why you might find that airline X has seats at (say) £300 on the flight, while airline Y is selling them for only £250;
(b) there will be no other cooperation between them - not even a frequent flyer partnership, so that you can't earn airline X FF miles if you buy tickets with the airline Y code on them, even if airline X is operating the aircraft.
The ultimate alliance is one in which the two airlines cooperate on things like marketing, pricing and selling. There'll be full reciprocity on frequent flyer benefits, and every flight will carry the codes of both airlines even though each airline operates only half the physical flights. The best example of this is not the Star Alliance, but KLM and Northwest.
Most alliances are somewhere in between these two extremes. For the regular customer, the biggest headline benefit is usually that you earn status across the entire alliance as if it were a single airline, and you are given recognition accordingly across the whole alliance.
It's true that the alliances are looking at joint purchasing of aircraft, but I don't think that any big deals have yet come off in this area. No airline will be required to take any particular aircraft, but all those looking to buy, say, A380s will club together and place one order. A big order gets a better price. Plus, in the Star Alliance's hopes, they will order aircraft to identical specifications, so that the aircraft can be more easily swapped/leased/sold to each other in the future.
If you want to look at the model for this, though, some individual airline groups have done it. For example, I think that BA placed a big common order for A319s which were intended to be distributed amongst different parts of the group, including franchisees that are legally and operationally totally independent companies.
Buying fuel, airport services, etc. and sharing terminal facilities is becoming much more common. When LHR T5 opens and hoovers up BA and a few other bits of oneworld, Star wants to move into a single terminal although rumours seem to be equally divided between T1 and T5.