A model is a 3D object. Models for OpenArena are usually created with Blender and then exported to MD3 format. Models can be placed inside maps (for example, as decorative elements), or it is even possible to work on a map as a model (see Modelling a map). Models placed inside any official OpenArena map are required to have a "source" available and to be released under the GPLv2 (or Public Domain) license, like everything else (see DeveloperFAQ#License).
Another software capable of working with MD3 models should be Misfit Model 3D (which has got "beta" MD3 support in its "development version"). Although it may not provide enough animation control for player models, it should be fine at least for static models (mapmodels to be baked into maps). That project has been halted, but another developed forked it and continued working on it as Maverick Model 3D. It has got improved MD3 support, there is a manual page about it here.
From the point of view of the final user, the most interesting kind of models are the character models, also known as player models, and often simply referred as models.
A "model" is a complete 3D character; while a "skin" is simply a texture variation (different colors, different symbols). More skins can fit the same model.
OpenArena comes with various models, and you can add more, specifically designed for OpenArena or for Quake 3 Arena. These would be contained in .pk3 files you have to copy into your "OpenArena/baseoa" folder.
You may be interested to:
- Select a model/skin: Manual/Player settings#Model
- Add a model designed for Quake 3 Arena: FAQ#How can I use maps, models, mods made for Quake 3 Arena?
- Know about the OpenArena characters: Manual/Characters, Characters, PlayerMDLs
- Create a model: Modeling, Making a player model, Creating a player model in Blender, MD3 format
- Other: Character ideas, Pak0.pk3/Models, Manual/Graphic options#Force player model, IffyContent
- ↑ In case of models designed for Q3, pk3 files smaller than 1 MB (just a rule of thumb) may contain just additional skins for baseQ3 models, instead of complete new models: in that case, they may not work, or look weird, when used in OA. Similar problems may theoretically happen also in case of additional skins designed for very different OpenArena versions (e.g. for very old OA versions).