The Standard core cylinders have a fixed middle (the key hole) that requires the removal of the entire cylinder for repair or re-keying. These are the most commonly used cylinders, since they are inexpensive. You can find these cylinders in most applications.
On the other hand, removable core cylinders are less common and tend to cost more at purchase time than the standard core cylinder. Removable core cylinders require the removal of just the core, which resembles a figure 8, for re-keying and most repairs. Removable cores allow easier re-keying as little hardware needs to be removed from the opening. This is cheaper than replacing the entire center like the standard core cylinders. During the removal of a core on a removable core cylinder, a special key, called a control key, is required to insert and remove the core from the cylinder housing. Removable cores are especially convenient for use in exit devices to prevent the need to remove the entire device from the door for re-keying. They are also recommended for use in institutions where frequent re-keying is common (such as apartment complexes, schools, sports stadiums, etc.).
Make sure you choose the correct cylinder depending on your application!