Advantage of getting your foot on the ladder without having to be able to afford a full mortgage, disadvantage you will still have to pay rent on the half you dont own but still cheaper than full mortgage. Is particluarly good in todays climate where getting a mortgage is very hard without a massive deposit.
The HA (housing association) own the other half you get a mortgage for half and then you can but more shares of the house as and when you can afford it. If you sell the property before owning all of it the HA get the percentage of what they own ie if you buy at 100k you mortgage 50k and rent 50k if when you sell its worth 120k you get 60k and they get 60k.
Normally can get on new build which means you have your own brand new home for less, very good scheme but you should check the contract from the HA to ensure you have the option to staircase your ownership to 100% should you want to, some HA's limit to 80% for instance so they will always get a share of the profit if you sell. Hope this helps!