For general questions about chords like how to play them, construct them, or what symbols represent them. For more in depth analysis of the ideas behind chords themselves, please use the chord-theory tag.
See also harmony, which is more general and covers notes being sounded together with different relationships, and the way those relationships change over the course of a piece.
Chords in a Western context are typically interpreted as "stacks" of major and minor thirds. Because of this, the name of a chord largely depends on the unique pattern of thirds beginning from the root note. The basic naming scheme also reflects this.
For example a "major" chord comprises, from bottom to top, a major third and a minor third. "C Major" consists of C (the root) E (a major third above C) and G (a minor third above E). Similarly, a "C minor" chord is built from C (the root) E-flat (a minor third above C) and G (a major third above E-flat. All "major" and "minor" chords, and others, follow these same thirds-based patterns.
There are other methods of building chords: for example, "quartal", where chords are built in fourths; and "quintal", where chords are built in fifths.