What are semantic roles?

          Semantic roles, also known as thematic roles, are one of the oldest classes of constructs in linguistic theory. Semantic roles are used to indicate the role played by each entity in a sentence and are ranging from very specific to very general. The entities that are labelled should have participated in an event. Some of the domain-specific roles are from airport, to airport, and depart time. Some of the verb-specific roles are eater and eaten for the verb eat. Although there is no consensus on a definitive list of semantic roles some basic semantic roles such as agent, instrument, etc are followed by all.

Examples of Semantic Roles

AGENT Agent is one who performs some actions. AGENT is a label representing the role of an agent. Joe played well and won the price. Here, Joe is the person who did playing.

CAUSE

Cause is one that causes something or it is a reason for some happenings. Rain makes me happy. Here, rain causes happiness and so is the cause.

EXPERIENCER

One who experienced is experiencer. Johan felt very painful when heard of the sudden demise of his friend. Here, Johan experienced the pain so he is the experiencer.

BENEFICIARY

I prayed early in the morning for Susan. Here Susan is the beneficiary.

LOCATION

Steve was swimming in the river. Needless to say river is the location.

MANNER

Tom behaved very gently even when he was insulted.

Here gently should be labelled as manner.

INSTR
Tom broke the wooden box with the hammer.

Here hammer is the instrument used to break the wooden box

Inner Roles

        
             To classify verbs, a distinction should be made between roles that are closely related to the verb and those that are not. For example most of the past tense verbs allow AT-TIME role realized by the adverb yesterday. Therefore AT-TIME is obviously more a property of verb phrases in general than a property of any individual verb. On the other hand, other semantic roles, those realized by constituents for which the verb subcategorizes seem to be properties of the verb. For example, the verb put subcategorizes for a preposition, moreover this preposition must realize the TO-LOC role. In verb classification this latter type of role is important, and these roles are called the inner roles of the verb. So for a given verb if the role is obligatory then it is an inner role. There are inner roles that are optional as well. All verbs may take at most one noun phrase in any given inner role. Multiple noun phrases must be related by conjunction. The following sentence is valid.
 
Jack and Jill went to the hill.
But the following sentence is invalid.
*Jack Jill went to the hill.

 Note that it is customary to put * before the invalid sentences.
 Similarly the following sentence is valid.
Jack went to the school and to the playground.
Here the conjunction and used to relate the two noun phrases.
But the following sentence is invalid.
*Jack went to the school to the playground.
So in the above valid sentences AGENT (Jack) and TO-LOC (to the hill, to the school) are the inner roles for the verb went.

Reference : http://language.worldofcomputing.net/semantics/semantic-roles.html

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