THEORIES OF MASS COMMUNICATION
Harold Lasswell's essential question is timeless (1949): "Who says what in what channel to whom with what effects?"

Theories are systematically related generalizations which attempt to explain and predict. 
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Lasswell's Early Theories (1920s and 1930s)

The Magic Bullet Theory
Media content tumbles through the air like a magic bullet penetrating the human psyche powerfully, uninhibited by other factors. WE are victims affected similarly, and uniformly. We cannot escape the crazed sex, drugs, and rock of MTV. We are powerless. the communicator is the causative agent. We are zombies and the magic bullet inescapably rules our being. So if we ban MTV our teen-age children will be safe.

The Hypodermic Needle Theory
A variation of the Magic Bullet Theory... Media content is injected from a giant hypodermic needle which represents TV or Rap music or films or whatever medium you are experiencing... It is a direct and powerful injection. The hypo injector medium is all powerful, the causative agent. Effects are substantial and uniform.
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Lazarsfeld's Two Step (later Multi-Step) Flow Theory (1940)
Opinion leaders or people who are looked upon as leaders or who are popular or who are more vocal get information, ideas, attitudes about things from mediated experiences and then in interpersonal situations they share what they have experienced from the media. Suggested that the interpersonal medium was more persuasive than any mass communication medium. Research was conducted on voting behavior in Erie County Pennsylvania.
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Festinger and the Consistency Theories (1950s)
People want their beliefs and judgments about things to be consistent with one another. In order to reduce dissonance created by inconsistencies in belief, judgments and action people expose themselves to information that is consistent with their ideas and actions, and they shut out other communications. For instance, suppose the Dems and the Repubs decided to use their conventions to persuade the other side that their candidate was the best. The problem? If I am a Dem I will probably not watch much of the Repub's convention (selective perception). 

If I am a Repub I will probably not watch much of the Dem's convention. Political conventions are for mobilizing the troops and the true believers not for persuading the other side. At least that is what this theory would suggest. Selective perception leads also to selective exposure and selective retention. That's why smokers will still smoke even when confronted with the most overwhelming evidence because what they retain will be the statement, "Well though the evidence seems overwhelming not all smokers contract lung cancer."
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McCoombs and Shaw Agenda Setting Theory (1970s)
The ability of the media to influence the significance of events in the public's mind ("the pictures in our heads). The media might not tell us what to think but they tell us what to think about. The media set the agenda for what we talk about. The media may mentally order and organize our world for us. When Madonna appears on Letterman and started using the F word (16 times) plenty of people talked about it the next day. 
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Katz's Uses and Gratifications Theory (1970s)
There should be less attention paid to what the media do to people and more attention on what people do with media. What are the personal and social uses to which people put media? Lull's television research illuminates this theory. He found that families used television for communication facilitation, relationship building, intimacy, and for structuring the day. 
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Stephenson's Play Theory
Play is an activity that is self sufficient and pursued for the pleasure in it. Stephenson says that his thesis is that the daily withdrawal of people into the mass media in their after hours is a step in the existential direction, that is, a matter of subjectivity which invites freedom where there had been little or none before. The fill of mass communication is not a flight from reality, escapism, or the like; nor is it debasing or seducing the masses as the critics suppose. Rather it is seen as a buffer against conditions which would otherwise be anxiety producing. Media are best regarded as communication-pleasure.
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Modeling Behavior Theory
Behaviors which are modeled from media experiences can become habitual if found useful and/or if they are reinforced in the environment. This is not about violent or criminal behavior.

This theory accounts for normal human behavior like the way we dress, phrases we use, the way we groom ourselves...
Stalagmite Theories

These theories suggest that mediated experiences induce long term effects that are very difficult to measure. The effects are like stalagmite drippings (drip, drip, drip) building up over time.
Meaning Theory and the Cultivation Theory are two of the most significant Stalagmite theories.
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Meaning Theory
Media experiences mold meanings by framing things in a particular way, by associating things together (beer and attractiveness), by presenting images that tell us what it's like to be a woman or a man or an African-American or an Irish-American or a teacher or a lawyer or a doctor or someone from Iowa. Media mold meanings with language too. Political correctness is in vogue because people realize how words generated by a mass medium can have an impact.
Does "NYPD Blue" present the real world of New York City police detectives? Does "ER" present the real world of Doctors? Questions like this are coming from a Meaning Theory focus on media.
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Cultivation Theory
Over time, particular symbols, images, messages, meanings become dominant and we absorb all this as true. Particular cultural stereotypes, ways of assessing value, hierarchies are established.
Take the outdated notion of contracts...
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Environmental Reinforcement
A crucial concept in the study of specific media effects. Generally, any behavior adapted from a mediated experience must include some form of environmental reinforcement to be long term.
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Postscript
At this stage in mass communication effects research the best we can say is what Berelson said: "Some kinds of communication, on some kinds of issues, brought to the attention of some kinds of people, under some kinds of conditions, have some kinds of effects." (1949)