Help me out here lads. I am trying to understand modern motorised infantry. In the US, would that be the stryker brigades? And what then is light infantry? The guys who drive to wrk in humvees? And what is their role in HIC?

Military Unit Organization of the United States (battleorder.org)

graylion said:

And what then is light infantry?

Such terms are usually organisation specific. Some times they are used out of tradition with the actuall equipment, function and organisation having nothing to do with it.

In general modern light infantry is a type of infantry that does not rely on vehicles. They can march, move and fight without. This has implications for their equipment. All of which needs to be man portable. Hence the term "light".
Its important to keep in mind that the fact that they don't need vehicles nowadays usually doesn't mean they don't have any. They are just able to leave them behind without significant reduction of their combat capability. Its also easily possible to airlift the entire unit because of this.

Which allready is on of the functions. High strategic and operational moblity by airlift. In a HIC scenario this can be used for all sorts of things.
Otherwise they are highly effective in difficult terrain. Terrain in which mechanised forces are limited. Dense forests, jungle, mountains and urban terrain. Light infantry is also usefull for screening and recon.