QoS stands for Quality of Service. It is quite an elusive term since there is no finite definition for it. Depending on where, how and why it is used, people see it in different angles and have different appreciations of it.
The most common definition we have of QoS is the differentiation between types of traffic and types of services so that the different types of service and traffic can be treated differently. This way, one type can be favored over another.
QoS is more in demand on corporate LANs, private networks and intranets (private networks interconnecting parts of organizations) than on the Internet and ISP networks. For example, you are likely to see QoS being deployed over a campus where students in dorm play half-life over the campus LAN, thereby congesting the network and hindering traffic for other more important types of data. QoS deployment in this case can favor traffic more the more important office data at the detriment of trivial network gaming, without however killing the latter. On the other hand, surfing the global Internet, there is most of the time no real QoS (unless your ISP has deployed QoS mechanisms). So, how quick you draw audio, text or video traffic generally depends on the bulk of the media. Text comes first, naturally. If your ISP provides QoS for, say, favoring voice, your voice reception would be great, and depending on your bandwidth, other media types might suffer.
QoS is an important tool for VoIP success. Through the years QoS mechanisms have become more and more sophisticated. Now, you can have QoS mechanisms for small LANs up to giant networks.
What is Quality?
In networking, quality can mean many things. In VoIP, quality simply means being able to listen and speak in a clear and continuous voice, without unwanted noise. Quality depends on the following factors:
Read more on VoIP voice quality: Factors affecting VoIP quality?
What is Service?
Service can mean many things in networking, as it carries some ambiguity in meaning. In VoIP, it generally means what is offered to consumers in terms of communication facilities.
As I mention so many times, the very first thing you need to guarantee in order to guarantee quality for VoIP is adequate bandwidth. And this is one of the greatest challenges in networks today: how to achieve good voice quality with limited and often shared bandwidth. This is where QoS comes into play.
Example: Your organisation deploys VoIP over a private LAN, which also accomodates other types of data - for surfing, downloading, faxing, and sometimes playing LAN games (especially when you, the boss, are not around) etc. You can take advantage of QoS to favor one of those classes of services over the others depending on your needs. For instance, if you want great VoIP quality, even if this means sacrificing other data types, then you can tweak QoS settings such that voice data is favored through the network.
VoIP Bandwidth Calculators
To be able to determine whether the bandwidth you have is fit for VoIP, you can have your bandwidth calculated. There are many places on the web where you can have this done for free, here for example.
How to Achieve QoS?
On a personal (small scale) level, QoS is set at router level. If you want to enforce QoS policies in your network, make sure you use a router which is equipped with QoS software, which you can use to configure the quality of service you require.
If you are an individual user, then there is a great chance that your VoIP service provider already implements QoS on their server, though this is not always the case. This way, the QoS configurations will be such that they favor voice over other data types. But then, since you will be using an Internet connection from a provider of another type (your ISP), the effect is somewhat diluted; unless you implement QoS on your ATA or router. Some IP phones allow this as well.