Why is CNF used for SAT and not DNF? I don't quite understand why almost all SAT solvers use CNF instead of DNF. It seems to me that solving SAT is easier using DNF. After all, you just have to scan through the set of implicants and check whether one of them contains not both a variable and its negation. For CNF, there's no simple procedure like this.
The textbook reduction from SAT to 3SAT, due to Karp, transforms an arbitrary boolean formula $\Phi$ into an “equivalent” CNF boolean formula $\Phi'$ of polynomial size, such that $\Phi$ is satisfiable if and only if $\Phi'$ is satisfiable.  (Strictly speaking, these two formulas are not equivalent, because $\Phi'$ has additional variables, but the value of $\Phi'$ doesn't actually depend on those new variables.)

No similar reduction from arbitrary boolean formulas into DNF formulas is known; all known transformations increase the size of the formula exponentially.  Moreover, unless P=NP, no such reduction is possible!