Authenticity is being true to oneself, keeping it real and having integrity with oneself.  Authenticity is the journey to a life based on the pursuit of your own purpose and guided by your own values, where we are true to ourselves without fear or compromise in all situations.
Authenticity is entirely compatible with modern living and even with having a job.  It absolutely does not require anyone to “drop out” or become a “hippy” (unless, of course that is who they authentically are) in fact it does not require anyone to do anything except to find out who they are and be it.
Authenticity is a process of evolving self knowledge and applying it to every situation and decision.
Wikipedia.org does not see it as being quite so clear or simple, here is what their entry on authenticity has to say:
“Authenticity is a technical term in existentialist philosophy. In this philosophy, the conscious self is seen as coming to terms with being in a material world and with encountering external forces, pressures and influences which are very different from, and other than, itself. Authenticity is the degree to which one is true to one’s own personality, spirit, or character, despite these pressures. Existentialists see this process in different ways.
It is difficult to determine the origin of the contemporary notion of authenticity. Writers on the subject of authenticity have often attempted to ground their views in work from a wide variety of historical periods. Secular and religious notions of authenticity have coexisted for centuries under different guises; perhaps the earliest account of authenticity that remains popular is Socrates’ admonition that the “unexamined” life is not worth living.
In the twentieth century, Anglo-American discussions of authenticity often center around the writers of a few key figures associated with existentialist philosophy, where the term originated; because most of these writers wrote in languages other than English, the process of translating and anthologizing has had a strong impact on the debate.
For these writers, the conscious self is seen as coming to terms with being in a material world and with encountering external forces and influences which are very different from itself; authenticity is one way in which the self acts and changes in response to these pressures.
Authenticity is often “at the limits” of language; it is described as the negative space around inauthenticity, with reference to examples of inauthentic living. Sartre’s novels are perhaps the easiest access to this mode of describing authenticity: they often contain characters and anti-heroes who base their actions on external pressures — the pressure to appear to be a certain kind of person, the pressure to adopt a particular mode of living, the pressure to ignore one’s own moral and aesthetic objections in order to have a more comfortable existence.
His work also includes characters who do not understand their own reasons for acting, or who ignore crucial facts about their own lives in order to avoid uncomfortable truths; this connects his work with the philosophical tradition.
Sartre is concerned also with the “vertiginous” experience of absolute freedom. Under Sartre’s view, this experience, necessary for the state of authenticity, can be sufficiently unpleasant that it leads people to inauthentic ways of living.
In all writers, authenticity is seen as a very general concept, not attached to any particular political or aesthetic ideology.This is a necessary aspect of authenticity: because it concerns a person’s relation with the world, it can not be arrived at by simply repeating a set of actions or taking up a set of positions. In this manner, authenticity is connected with creativity: the impetus to action must arise from the person in question, and not be externally imposed.
Heidegger takes this notion to the extreme, by speaking in very abstract terms about modes of living; his terminology was adopted and simplified by Sartre in his philosophical works. Kierkegaard’s work (such as the “Panegyric Upon Abraham” from his Fear and Trembling) often focuses on biblical stories which are (naturally) not directly immitigable. Sartre, as has been noted above, focused on inauthentic existence as a way to avoid the paradoxical problem of appearing to provide prescriptions for a mode of living that rejects external dictates.
These considerations aside, it is the case that authenticity has been associated with various human activities. For Sartre, Jazz music was a representation of freedom; this may have been in part because Jazz was associated with African-American culture, and was thus in opposition to Western culture generally, which Sartre considered hopelessly inauthentic. Theodor Adorno, however, another writer and philosopher concerned with the notion of authenticity, despised Jazz music because he saw it as a false representation that could give the appearance of authenticity but that was as much bound up in concerns with appearance and audience as many other forms of art.
Heidegger in his later life associated authenticity with non-technological modes of existence, seeing technology as distorting a more “authentic” relationship with the natural world.
Most writers on inauthenticity in the twentieth century considered the predominant cultural norms to be inauthentic; not only because they were seen as forced on people, but also because, in themselves, they required people to behave inauthentically towards their own desires, obscuring true reasons for acting.
Advertising, in as much as it attempted to give people a reason for doing something that they did not already possess, was a “textbook” example of how Western culture distorted the individual for external reasons. Race relations are seen as another limit on authenticity, as they demand that the self engage with others on the basis of external attributes.
An early example of the connection between inauthenticity and capitalism was made by Karl Marx, whose notion of “alienation” can be linked to the later discourse on the nature of inauthenticity.
Hence those concerned with living authentically have often led unusual lives that opposed cultural norms; the rise of the counter-culture in the 1960s in Europe and America was seen by many as a new opportunity to live an authentic existence. Many, however, have pointed out that just because one lives unusually, one is not necessarily in an authentic state of being.
The connection of the violation of cultural norms to authenticity, however, is strong and real, and continues today: among artists who explicitly violate the conventions of their profession, for example. The connection of inauthenticity to capitalism is contained in the notion of “selling out,” used to describe an artist whose work has become inauthentic after achieving commercial success and thus becoming to an extent integrated into an inauthentic system.
If authenticity can only be described in very abstract terms, or as the negative of inauthenticity, what can be said about it directly? All writers agree that authenticity is:
Something to be pursued as a goal intrinsic to “the good life.”
Intrinsically difficult, due in part to social pressures to live inauthentically, and in part due to a person’s own character.  A revelatory state, where one perceives oneself, other people, and sometimes even things, in a radically new way.
One might add that many, though not all, writers have agreed that authenticity also:
• Requires self-knowledge.
• Alters radically one’s relationships with other people.
• Carries with it its own set of moral obligations.
• Can be obtained regardless of race, gender and class.”