Hi cuddleMe,
I currently live in a shared ownership house so I can give you a rough outline of how it works with the housing association I am with.
You decide first of all how much of the property you can afford between 25 and 75%, subject to agreement by them obviously, you then secure a mortgage, say for example for 50% of the market value of the house. The owner ( usually a housing association ) owns the rest of the property and you then are responsible for paying them rent for their share.
This can work out cheaper than renting depending upon the rent set out by the owner. I certainly saved about �150 per month as opposed to renting in my area.
After a certain amount of time you are then able to purchase more of the property from the owner and then your rent would subsequently be less.
I think shared ownership is a marvellous idea for people that simply cannot afford to get on to the housing ladder, certainly helped us!