Fitted Diapers have the fasteners built right in.  They do require a cover to be water proof or can be used without around the house so you know when baby needs a changing.  They usually stay put better than other diapers since they are fitted and usually absorb a great amount of liquid.  I love using a fitted diaper with a wool cover for over night.  Fitted diapers usually keep your covers cleaner since they keep messy poops more contained in the diaper and because of this make it possible for baby to go without a cover more easily if you prefer.  You would need about the same amount of fitted diapers as pockets or AIO about 12 diapers if this is all you were going to use.  Some like to just use these for over night and use other diapers during the day so they may only have 2 – 4 fitted diapers.