The purpose of this short article is to explain the use of titles of nobility for genealogical purposes.
A title of nobility is by no means subject to international standardisation, but rather to national and local differences.
A full treatment of the subject can be found at
http://www.heraldica.org/topics/odegard/titlefaq.htm.
Here it will suffice to say that the title of nobility you are most likely to come across in the English-speaking world is, in descending order: Prince, Duke, Marquis, Earl/Count, Viscount, Baron and Noble. If your family originally comes from a non-English-speaking country, you will need to search for a possible title of nobility in the relevant language. At the web address above you will find information about titles of nobility in various languages.
A title of nobility is to be understood as a rank, similarly (and in some respects corresponding) to military ranks. Usually the noble rank is indicated by a coronet (“crown”) on the coat of arms, with one specific coronet signifying each rank. The coronets may have a different number of pearls or leaves to indicate each rank, etc. The customs are different in different countries.
A title of nobility, as it is used today, is in most cases honorific, that is, it does only carry honorific privileges. These honorific rights include the rights to be recognised as a nobleman / noblewoman and belonging to the class of the nobility, the right to (where appropriate) use the title in question, the right to use noble arms with proper noble insignia, and the right by common law to certain appropriate predicates.