What is the difference between hypnosis and meditation? It seems that many people say that hypnosis and meditation are very similar. But what are their key differences (both in a soft and hard way)?
I believe this question is impossible to "answer" because the inherent premises in the question are based on the assumption that there are two discrete dichotomous phenomena, "hypnotism," and "meditation," which can somehow be meaningfully "defined," and, then, "compared."

Both "hypnosis" and "meditation" are figuratively vast conceptual collages of imprecise multiple-meanings, observations of what appear to be "special" states of human awareness, and consciousness, admixtures from what in "western science" is termed "psychology" with an equally vast mix of practices, lumped-together as "spiritual," from the world's religions, etc.

From the viewpoint of epiphenomenal philosophy ... wikipedia entry ..., and from some modern, western, theorists of human consciousness as grounded in our biological, neurological, reality, both hypnosis and meditation can be regarded as secondary phenomena: in the sense that they are culturally defined and shaped modes of thought, behavior, and experience, in a "continuum of consciousness," which, over time, are conserved, institutionalized, even valorised into being perceived as "sacred."

In Vipassana meditation, in the Theravadan Buddhist tradition (Sanskrit "sthaviravada," "teachings of the elders") one way the "anti-goal" of meditation could be defined is: it is a practice meant as a vehicle for the de-hypnosis of the "illusory personal, small, self," leading to a direct experience of the "real, undifferentiated, trans-personal Self."

But, in that tradition, in its "purest" form, meditation is not a "goal in itself," and meditation is not a "means to an end" of achieving "altered states of consciousness."

But, there are countless other types, schools, philosophies, of "meditation," with very different conceptual underpinnings, some "secular," some "formally religious," and a wide range of behavioral practices associated with them.

On the "grossest" level, you might say we observe, when we see what appears to be hypnosis of one person by another, a loss of conscious, and subconscious, control of self from the person hypnotized to the hypnotist. But, then: there's stage hypnosis, self-hypnosis, so-called therapeutic hypnosis for behavior modification (such as giving up smoking, etc.).

While we typically associate meditation with acts of what we define, in western psychological terms, as individual "self-control," and "discipline," there are traditions in which group practices (rituals) leading to mass ecstasy are considered meditation (Sufi Zhikr ?). 

From one perspective, "cultural identity," itself, can be considered a cohesive set of internalized hypnotic states of mind: continually reinforced from within, and without, the self, and modified in biological development throughout the life-cycle ... I am not referring, here, to the movie-series, "The Matrix" :) !

Suggested readings: 


Hypnotism and psychotherapy: Milton Erickson ... Amazon page ....
Psychology and consciousness: books by Robert Ornstein ... Ornstein's website ...; books by Andrew Weill ... Weill's Amazon page ..., books by Steven Pinker, particularly, "The Language Instinct" ... Pinker's website ....
Meditation and Buddhism:

a. Donald Lopez ... "The Story of Buddhism: A Concise Guide to Its History &amp; Teachings" ...: excellent overview of how original Buddhism became "modified:" incorporated a mixture of Vedic/Hindu classic ideas (influence of Advaita Vedanta, and Yogacara schools, for example); incorporated Sivaite Tantra as developed in what is now Bengal; and, later, became "deistic," with an elaborate hierarchic pantheon, in India, Tibet, China, Japan (into what can be loosely lumped together under the rubric "Mahayana Buddhism"). Depending on your biases, you might say, rather than "modified:" "evolved," or "corrupted.."

b. Theravadan Buddhist tradition (Sri Lanka, Southeast Asia), and Vipassana meditation (aka "mindfulness", or "insight" meditation): Walpole Rahula ... "What the Buddha Taught: Revised and Expanded Edition" ...; Thich Nhat Hanh ... Amazon page ...; books by Joseph Goldstein ... recommended: "Insight Meditation: The Practice of Freedom" ...; books by Bernard Kornfeld ... Amazon page ....