An equilateral triangle is one in which all three sides are congruent (same length). Because it also has the property that all three interior angles are equal, it really the same thing as an equiangular triangle. See Equiangular triangles.
An equilateral triangle is simply a specific case of a regular polygon, in this case with 3 sides. All the facts and properties described for regular polygons apply to an equilateral triangle. See Regular Polygons