A glowing texture is one that does not require another form of light to be lit up. It can create its own lightmap independent of all lighting sources and may even create light itself. This does not mean that the surface will appear pure white, just that the face will ignore any light it would receive. This is a key feature as a "glowing" black stays black and does not get brighter. The face achieves what is called fullbright, where the texture's pixels appear exactly the same color as they were saved.
There are several different ways to create this effect available in the Source engine.
For glowing textures on models or brush faces:
$selfillum parameter, specified in a material's VMT. - Use this for most purposes, not compatible with transparency.
UnlitGeneric shader, specified in a material's VMT - Does not emit light, forces face to ignore all lighting, making it fullbright.
$detail parameter with $detailblendmode value of 5 or 6, specified in a material's VMT - Same as $selfillum, but more rudimentary and compatible with transparency.
Using info_lighting to fool a model into thinking it is brightly illuminated.