A dodecagon is a polygon with 12 sides, or edges. This is true whether the dodecagon is regular or not. A regular dodecagon is one with 12 sides of equal length. An irregular dodecagon is one with 12 sides that are not all of equal length. According to Wolfram MathWorld, which claims to be the web's most extensive mathematical resource, this includes a Latin cross.
A dodecagon has 12 angles. In a regular dodecagon, every internal angle is equal to 150 degrees. This means that the combined total of the internal angles is 1,800 degrees. In an irregular dodecagon, the internal angles are not necessarily 150 degrees and are not all equal. In a Latin cross, for example, 8 of the internal angles are right angles, or angles of 90 degrees, and 4 of the internal angles are 270 degrees. However, the total is still 1,800 degrees.
The external angles of a regular dodecagon are all 210 degrees. The total of all the external angles is 2,520 degrees. The external angles of an irregular dodecagon are not necessarily 210 degrees and are not all equal. In the Latin cross, for example, 8 of the external angles are 270 degrees and 4 are 90 degrees. However, the total is still 2,520 degrees.
A vertex, in the case of a dodecagon, is a corner point, where two sides or edges meet. The plural of "vertex" is "vertices". Both regular and irregular dodecagons have 12 vertices. In a regular dodecagon, all the vertices have the same properties. In an irregular dodecagon, at least some of the vertices have different properties from some of the others.