NewGRFs aren't necessarily just 'art packages'; they can do a lot beyond just visual stuff, and many don't even contain any art at all.
Depending on the grf, they can do all sorts of things:
- Add vehicle types. (probably the most common).
These often have more features than the default vehicles, for example:
- Support for non-default cargo types
- Speed limits for wagons, not just locomotives
- Support for 'autorefitting' (refitting in stations).
- Different graphics depending on date, or the type of wagons being pulled, etc.
- Maximum ranges for aircraft.
- Trams (the default vehicles don't include any).
There's a slight variant:
- Modify the existing vehicles directly, changing the stats or introduction dates.
- Modify infrastructure.
- Add new types of town buildings.
Mostly just for aesthetics, but it's possible for such sets to control the rate of town growth.
These can be used together, but sets trying to do 'clever stuff', e.g. TaI, won't be able to do that when used with others.
- Add station types.
These are mostly just artistic. Some display the amount and type of cargo waiting, which can be useful.
All can be used together, I think.
- Add new types of railway.
This includes types of track with speed limits, or third-rail electrified tracks, or narrow gauge.
Most such grfs remove the default game's Monorail and Maglev tracks.
These can be used together to some extent, but there's a quite restrictive limit on the number of different track types that can be defined. If you use a combination of sets that between them define >16 types of track, it won't work properly.
- Change the appearance of roads.
No gameplay effect; some sets might display different road graphics depending on date.
These can't be used together. Also, they don't alter the road surface on bridges.
A variation on this are tramtrack grfs, these change the appearance of tramways.*
- Change the bridge types.
These replace the default bridge types, and can have slightly more types of bridges than normal.
They can't be used together. Also, the road surface on the bridges doesn't change to match roadsets; some bridge grfs have alternate versions to match various roadsets.
- Replace airport and dock graphics; allow rotatable airports.
For dock graphics, last one in the list wins. I don't know what happens with airports.
- Add new types of town buildings.
- Modify industries and cargotypes
GRFs can modify the properties of the default industries and cargos, or define additional or replacement ones.
These often have different mechanics to the default ones, for example FIRS has a 'supply' mechanic, and ECS and TaI both have a 'stockpiling' system. Never use multiple industry grfs together unless they're specifically designed to work as a group.
- Modify the landscape.
- Change the appearance of landscape tiles.What it says on the tin, really. For obvious reasons, one such grf at a time.
- Add NewObjects.
Purely for looking at, they add buildable tiles under the Landscaping toolbar. Harbour walls, trees, wind turbines, flocks of seagulls...
These can all be used together.
- Modify basecosts.
Change the price of terraforming, building, bribes...
The effects of these are cumulative, IIRC
There are one-off (so far) grfs that do other stuff. Someone remind me if I forgot any significant categories, please.
Remember that a single grf can do any combination of these things - for example, several sets that add rail stations also replace dock and shipyard graphics, some townsets replace road and bridge graphics, there's one trainset (NARS) that actually defines a cargotype and has been known to conflict with ECS industries. Most are reasonably self-contained, though.