The Citadel miniatures used to play Warhammer 40,000 are simply referred to as models in the rules that follow. Each model is an individual playing piece with its own capabilities. It is convenient to define different types of models for ease of description throughout the rules.

There are two very general types of models. These are:

This category includes an enormous variety of troops ranging from Gretchin, the small, weak slaves of the Orks, to Wraithlords, mighty Eldar constructs containing the souls of the dead, Non-vehicle models are the most common in the game and all use a standard profile described in the Characteristics section.

Needless to say, this includes all vehicles. In practice it includes all vehicles that enclose the crew Small vehicles, such as bikes, are not included, instead they are dealt with as an extension of their rider. All vehicles will have a profile described in the Vehicles section
When you see the term 'model' used in the rules, it will apply to both non-vehicle and vehicle models. If the term 'non-vehicle model' or 'vehicle model' is used it will apply to models using characteristics or vehicle characteristics respectively. To ensure that you can recognise the difference it is probably a good idea to take a quick look at these sections now.
Models will sometimes fight individually. This is commonly the case with powerful tanks or experienced army commanders. Normally, though, a number of models are combined in a group termed a unit. The different types of unit are detailed in the Units section.

There are countless different models in Warhammer 40,000 - everything from towering daemons and immense war engines to tiny, crawling vermin. To ensure no one gets confused by the huge variety of models in the game, a couple of conventions apply to all models on the tabletop battlefield.

Firstly, a model is considered to occupy the area of its base, so when measuring distances use the closest edge of the base as your reference point. For models without a base (usually vehicles) use the model's hull/main body instead. Games Workshop miniatures are usually supplied with a base and this should be viewed as the minimum size base they can be mounted on - you can mount them on something bigger if you wish, but not something smaller.

The rules will often have to take account of the height of models and terrain features. This is necessary to determine whether troops can see over obstructions. This does not mean literally their actual height, as the simple expedient of crouching, kneeling or crawling will render such direct comparisons irrelevant. Instead there are three broad height bands into which all models fall. These categories are also used to define the height of some terrain features. The following three categories are the only ones that are important in the Shooting phase when determining line of sight and target priority:

Size 1: Small Targets
Size 2: Standard Targets. Basically every model not included in either of The other categories.
Size 3: Large Targets. Includes Monstrous Creatures and Vehicles (see Units).
Initially, virtually every model you use will fall into the middle category. Be aware though that when you want to see over some terrain features or an ongoing close combat, these heights will become relevant. All you need to remember is that if an observer OR the observed is of a greater height category than anything in the way, then it has a clear line of sight.