Together[expr] puts terms in a sum over a common denominator, and cancels factors in the result.
Example: Together[1/x + 1/(1-x)] .
Together makes a sum of terms into a single rational function.
The denominator of the result of Together is typically the lowest common multiple of the denominators of each of the terms in the sum.
Together avoids expanding out denominators unless it is necessary.
Together is effectively the inverse of Apart.
Together[expr, Modulus->p] generates a result modulo p.
Together[expr, Extension->Automatic] allows operations to be performed on algebraic numbers in expr.
Together[expr, Trig -> True] treats trigonometric functions as rational functions of exponentials, and manipulates them accordingly.
See The Mathematica Book: Section 1.4.5 and Section 3.3.3.
See also: Cancel, Collect, Factor, PolynomialGCD.