Restorative Dentistry is the restoration of teeth to proper function and
health.
Fillings - When a tooth has been infected with decay, the decayed
tooth structure needs to be removed.  Then, once it is removed, the
tooth can be restored to proper function.  This is the basic concept
behind fillings.
Crowns - A crown is done to cover the entire remaining tooth structure.  
There are many reasons for the need to do a crown.  For example, if
there is very little tooth structure remaining after removing decay from
a tooth, the resulting filling would be large and the chances of
fracturing the remaining tooth are high.  So a crown is done to
strengthen the tooth.  Another reason that crowns are done is when
there is a crack in the tooth.  They can also be done for
cosmetic
reasons.
Bridges - A bridge, or fixed partial denture, is a means to replace a
missing tooth by preparing the teeth on both sides of the space for
crowns.  So that the space is filled with a crown that is attached to two
crowns on bot sides of it.
Restorative Dentistry