Open Bearings are normally loose balls or with a cage that then run in a 'race' or set of 'races'. They can be easily stripped down, removed, cleaned and regreased. The problem is as they are 'open', they are more susceptable to the ingress of dirt, grit, water etc all of which can have a detrimental effect to both balls and race.
Sealed Bearings are usually where the balls and races come as one unit, pre-packed with grease (supposedly) and sealed buy a rubber or plastic seal. These are less susceptable to the ingress of foreign objects, but are designed to be disposable items (ie not designed to be stripped down). They can still be removed, cleaned and repacked with grease, although you will have to remove the seal to re-pack and then they won't be sealed any longer.
Sealed bearings are seen as the standard due to their resistance to foreign object ingress, although a a meticulously maintained set of open bearings can work just as well