Second Stage Regulators reduce the pressure delivered from the first stage (also called the intermediate pressure - ca. 10bar) to the surrounding pressure (ambient pressure) around you, allowing you to breath comfortable underwater. You will find, similar to the first stage, different options depending on your type of diving.
Balanced and Unbalanced
You will balanced and unbalanced second stages on offer. The concept is very simple, a balanced second stage, will adjust its performance and will give you a consistent breathing resistance regardless of depth, while an unbalanced one cannot do this. An unbalanced regulator will either have a "too easy" breath at surface or a "harder breath" at depth. However, this is not as critical as it sounds, many regulators have the ability to make adjustments as depth increases and if you are a recreational diver, who will not venture often beyond the 30 metre mark it is only of limited consequence.
Inhalation Adjustment and Venturi Control Options
You will find a second stages fitted out with little adjustment buttons, you effectively can adjust the breathing resistance whilst diving. There are in principle two types of adjusters. The Venturi Switch (usually a lever) marked with "+" and "-" (pre-dive) symbols. This effectively increases the valve pressure inside your second stage, increasing the effort to open the valve - this decreases the risk of free flow - when needed you can still breath from it and pull the lever to make it easier to breath. The second alternative is the "Inhalation Adjustment", often a little knob (often embedded around the Venturi control lever), allowing you to adjust the breathing resistance while diving (this is typically seen on balanced second stages). If you intend to do a lot of mixed diving (esp. cold water diving) - this is a worthwhile feature to have.
Alternative Air Source - Redundant Second Stage - Octopus
You will find on your regulator set usually a brightly coloured second stage (bright yellow is typical). This can be used by yourself, if your primary second stage fails, or your buddy requires an "alternative air source". The brightness allows for easy recognition by yourself or your buddy. It is recommended you have at least the same performing regulator on it, because in an emergency you are likely to require a better performance as you or the victim will be breathing harder.
You will find some divers will only have one seconds stage regulator and their second "alternative air source" (AAS) will be attached to their BCD inflator (see photo on the right). This option reduces the weight of your kit (especially for travelling), but has the drawback that the Inflator hose is quite short, which requires you to first remove your primary regulator and give it to your buddy. You then will need to use your BCD AAS to breath and control your buoyancy - which means it does require a bit of practice as you are much more movement restricted.
Nitrox / Mixed Gas
Most regulators are able to take Nitrox (Enriched Air) up to 40% O2 without any modifications. However, do check the owners manual to be sure. Most regulators will require special O2 cleaning and the use of O2 grease, before a richer gas over 40% O2 can be used. Especially lightweight regulators which use special materials (such as Titanium) are often not capable of using enriched gases of higher percentages. This is one reason why technical divers tend to have quite chunky equipment for their diving.
In recent years, manufacturers have been experimenting with use of advanced lightweight materials - Titanium, ceramics, special alloys to name a few. These are often regulators designed for travelling, especially attractive with the ever decreasing weight allowances from airlines. If you intend to dive in the UK, you must check their ability to be used in waters below 10°C, as many lightweight regulators are not cold water rated. However, some of the very expensive lightweight regulators (often using carbon technology) have been able to meet the strict requirements for cold water diving.