Outer layers are a very important layer and often the most technical.
This is the layer obviously exposed to the elements, and therefore it might need to be waterproof and/or windproof as well as folding up small enough to easily fit into a pocket.
Here specifically over the last few years with the development of modern synthetic fibers we have seen a huge jump in the development of these items.
Waterproof jackets are now extremely technical pieces of cycling clothing and can often be one of the most expensive. Important things to look out for.
Different jackets have different levels of waterproofness for showers or sustained heavy rain, you should look to see if the zips are covered and if the internal seals are taped. If you see that you might be riding all day in bad weather then a heavier waterproof with taped seams and possibly a hood will keep the water out for way longer.
Recent advances in materials have allowed the construction of cycling garments that allow one way movement of moisture, so sweat will wick out but rain cannot come in. This is a huge plus for many sports but cycling in particular benefits from a jacket that keeps you dry but allows you moisture to escape when you are sweating and working hard.
Again another particularly important characteristic for cyclists is can the jacket be folded small enough to be carried in a jersey pocket. As normally everything must be stowed on the rider or on the bike a lightweight comfortable water proof that is also effective when the weather turns is a great addition to any cyclist's wardrobe.
A gillet which can be very compact kept in a pocket in case of showers during the ride, where a long sleeve waterproof is not required. Also this type of windproof item is great if you have a long descent on the ride.
Other designs for outer layers can include neoprene jackets that are splash proof but not fully waterproof but for cold, dry days provide the best insulation. One other consideration is colour, a very bright waterproof or overlayer used probably in bad weather when visibility is poor can help make sure you are seen out on the road.