The iron triangle is a project management concept that states that project scope, time and costs (or resources) are interrelated. The iron triangle represents these three vectors as three sides of a triangle:

1. Scope - Features, Functionality and Quality
2. Time - Schedule and work time allotted
3. Cost/Resources – Internal staff time, consultant fees, expertise available, assets and equipment required

Often times projects fail because one of these vectors is changed without a change in either of the other two.

The iron triangle is embodied in the old consultant saying:
"you can have it done fast, cheap or right, pick two (or pick one).”

Humorous Sidebar

Wikipedia lists several other plays on the iron triangle:

College: Work, Sleep, or Play – Pick two.
Girls: Single, Sane, Attractive – Pick two.
Men: Handsome, High-Earner, Faithful – Pick two.
Choose two: (A) Fast, (B) Efficient, (C) Stable, (D) Windows (counts as two)

Additional Reading

The broken iron triangle is broken