When you are purchasing a property with another person this question must always be answered. The two concepts of property ownership can have major repercussions if there is some event that creates a need to sell or dispose of the property in question!
Ownership of property as Joint tenants is such that each owner has a legal and beneficial interest in 100% of the property jointly with the other co-owners. When one co-owner dies, their interest passes to the other co-owner. Generally speaking this is the type of ownership a married couple would seek when purchasing a property. It operates on the principle of survivorship.
Ownership in Common or as Tenants in Common is where each co-owner has a specified share of the ownership of the property. Normally this would be divided as a half share each, but it can be in whatever share is agreed to by the co-owners. This is used in circumstances where perhaps friends purchase a property and each individual wishes to have a specified share in the ownership of the property.
In the case of individuals purchasing a property as Tenants in Common it is always advisable to have a co-ownership agreement drawn up. It is also always advisable that individuals who purchase a property as Joint tenants or tenants in common should also complete a will. When you are purchasing a property in a situation where you are not purchasing as husband and wife then it is always necessary to discuss prior to signing the contracts for sale what type of ownership you are looking to have after the completion of the conveyance on your behalf.