What Is EVOLUTION All About?

Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered.
Moreover, "fact" doesn't mean "absolute certainty"; there ain't no such animal in an exciting and complex world. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are not about the empirical world. Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us falsely for a style of argument that they themselves favor). In science "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional consent." I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.
Evolutionists have been very clear about this distinction of fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: Establishing the fact of evolution, and proposing a theory--natural selection--to explain the mechanism of evolution.

I agree with most of what peach bug said, but would like to also point out that evolution is not just a scientific theory - it is a proven fact.  Scientific evidence undeniably shows that organisms evolve over time.  There are, however, several theories concerning the specific processes by which evolution occurs (i.e., natural selection, evolutionary fitness, etc.).

Evolution is, in it's most basic form, the belief in changes over time. It is the most widely accepted scientific theory as to the current state of biology today, leading to it's widespread teaching in schools. There are two main misconceptions about evolution: That it means humans evolved from apes and that theory means random guess. The ape/human misunderstanding is actually derived from the evolutionary belief that man and ape have a common ancestor. Like the Galapagos finches, distant ancestors took very different roads to development. There are two mainstream types of possible evolution; those in leaps and steps. Leaps would be a belief that the development of wings took only a few hundred years because of drastic mutations, while steps meant thousands to millions. There is evidence of both in today's flora and fauna. Theory means tested, observed, and evaluated, and the title theory is given to conclusions for which it is the best scientific explanation available. There is evidence and it is very strong, whether or not a non-scientific public protests...

The definition
Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.
The explanation
Biological evolution is not simply a matter of change over time. Lots of things change over time: Trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent through genetic inheritance.
The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.
Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: Humans and oak trees, hummingbirds and whales

It's about gradual change in the characteristics of living things over generations esp..to a more complex form

I'll deliver what i once read in an encyclopaedia as follows-
it is said that first living organisms developed in water. It was due to lightening in the sky which fell on an ocean and eventually due to some reactions developed a microorganism!
Then from this single celled organism to a multi cellular organism which ultimately lead to the birth of fish! But these fishes had to come to the surface of water to get oxygen so they'd often spend their time on shores which led to the formation of mermaids (but not with a human face but a weird face of some animal)and then ultimately led to development of animals! And how humans came from animals , i think i needn't tell!!