Ok, I’ll try to newbify my explanation:
models (usually .md3 or .ase) - objects like the truck, the tank, command posts, construction crates etc. These are usually complex objects that would be difficult to make simple using brushwork. So, a 3-D modelling program such as 3DMax or gmax are used (search the web on more info on 3-D modelling programs). Models are then used in ET as misc_models or misc_gamemodels (do a search on there forums for information on the differences between the two). Other models that appear in game are trees, bushes, most of the light fixtures, the faucet of the sink, etc. Check in etmain/models/mapobjects for the ET models most commonly used
hard-coded - part of the executable which cannot be modified without modifying the source code. ET.exe is the executable. It has variables that can be changed without modifying ET.exe – things like loading different maps, all the cvars, mouse controls etc. ET.exe also has things that cannot be changed without creating a mod.
mod - modification to the ET.exe source code that changes things otherwise unchangeable.
mg42 - an entity in ET that has certain properties. Some of these properties can be modify in a map, others cannot. The mapper can determine accuracy, horizontal and vertical firing arc, health. But the mg42 will behave in certain ways (fire bullet, overheat, etc) due to the entity definition in ET.exe.
Advice: read the LDR (multiple times), read the sticky: Tutorials and Mapping Resources, search the forum. Most of these questions have been asked over and over and over - and there are much better explanations than I can provide.