Individual Leases: Most common for apartments for university students. Each room in the house is leased separately. You may be matched with other students who you have not met. Each roommate is legally responsible for their individual rent. So if the apartment costs $2000 a month but your room costs $500 a month you are only responsible for your payment of $500. This is great for students as you are not liable if your roommates fail to pay the rent.
Another advantage to individual leasing is that it is usually significantly cheaper to rent a room in an apartment with many rooms than an apartment with one room. This is because you will be sharing common areas and bathrooms as well as expenses such as gas and heating.
Conventional Leases: When an apartment is rented out as a total unit. There is only one lease signed rather than each roommate signing an individual agreement. Each residents listed on the lease is responsible for the terms of the lease. This means if your ex-BFF leaves the apartment without paying the rent, you would be responsible for the monthly rent on the entire apartment.