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I'm relatively new to Blender and mainly use it for viewing 3D geometry in various formats (such as OBJ). Currently I'm writing my own mesh voxelizer, and my program has support for outputting the resulting voxel array to an OBJ file containing a cube for each voxel. It's not incredibly effective, but I found it to be useful for quickly checking whether or not my voxelizer produces adequate results.

I have a three-dimensional voxel grid; every voxel that is a 1 (instead of 0) is represented as a simple cube, and all these cubes are stored in an OBJ file. The OBJ file contains only the vertex positions and face definitions, there's no MTL mentioned in the OBJ file (and no MTL file is generated, accordingly). Here's an excerpt from a typical OBJ generated by my program:

v -1,20000004768372 -1,20000004768372 -1,19999957084656
v -1,20000004768372 -1,20000004768372 -0,899999618530273
v -1,20000004768372 -0,900000095367432 -1,19999957084656
v -1,20000004768372 -0,900000095367432 -0,899999618530273
...
f 1 2 3
f 2 4 3
...

When I open it in Blender, I see that all geometry is correct, but one triangular face (always the first one in the list of faces!) is highlighted differently by Blender. Here's a screenshot (imported the OBJ, selected the whole mesh, didn't touch anything else):

One triangle is highlighted differently

(It's not a representation of any meaningful geometry yet, just a randomized voxel grid, for those wondering)

I'm not sure what Blender is trying to tell me here. How is this face any different from all the other ones (except for being the first face in the file)? What is wrong with the OBJ file my program generates? There's no color assigned to vertices or faces, there's no material specified, all vertices and faces should be the same.

Here's the file in question, if you want to examine it:

https://www.sendspace.com/file/wlrva9 (171 kB)

It's actually a re-created file so the random cubes might be different here and there, but the problematic face in question is still there: look for the corner vertex at (-1.2, -1.2, -1.2).

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That is simply the active element. The active element changes all the time when you change the selection. It is usually the last element you selected.

It is used for various types of things. For example, the most common use is if you have the transform pivot point set to active element, it will use the active element as the center of the transformation. You can change it to another element by selecting that piece of geometry last.

In object mode, the active element will be an object, it will have a slightly lighter orange boundary around it. It is more significant in object mode, since when you change most settings in the properties panel (or go into edit mode) it will only affect the active object.

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As pgmath said, this is the active element. It is mostly used as a reference for blender. Operators usually use it as a special pivot point by changing the pivot to active. Also may be used in union with snapping.

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