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Constituency Parsing is the process of analyzing the sentences by breaking down it into sub-phrases also known as constituents. These sub-phrases belong to a specific category of grammar like NP (noun phrase) and VP(verb phrase).
Dependency Parsing is the process to analyze the grammatical structure in a sentence and find out related words as well as the type of the relationship between them. Each relationship: Has one head and a dependent that modifies the head.
Constituency Parsing. The constituency parse tree is based on the formalism of context-free grammars. In this type of tree, the sentence is divided into constituents, that is, sub-phrases that belong to a specific category in the grammar.
Dependency parsing helps us build a parsing tree with the tags used determining the relationship between words in the sentence rather than using any Grammar rule as used for syntactic parsing which gives a lot of flexibility even when the order of words (like 'boy handsome' or 'handsome boy') get changed.
A constituency parse tree breaks a text into sub-phrases. Non-terminals in the tree are types of phrases, the terminals are the words in the sentence, and the edges are unlabeled. For a simple sentence "John sees Bill", a constituency parse would be:
Sentence
|
+-------------+------------+
| |
Noun Phrase Verb Phrase
| |
John +-------+--------+
| |
Verb Noun Phrase
| |
sees Bill
A dependency parse connects words according to their relationships. Each vertex in the tree represents a word, child nodes are words that are dependent on the parent, and edges are labeled by the relationship. A dependency parse of "John sees Bill", would be:
sees
|
+--------------+
subject | | object
| |
John Bill
You should use the parser type that gets you closest to your goal. If you are interested in sub-phrases within the sentence, you probably want the constituency parse. If you are interested in the dependency relationships between words, then you probably want the dependency parse.
The Stanford parser can give you either (online demo). In fact, the way it really works is to always parse the sentence with the constituency parser, and then, if needed, it performs a deterministic (rule-based) transformation on the constituency parse tree to convert it into a dependency tree.
More can be found here:
http://en.wikipedia.org/wiki/Phrase_structure_grammar
http://en.wikipedia.org/wiki/Dependency_grammar
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