Our eye can see a lot of colour range.
But all devices can not show that much range of colour. That's why we mapped colours mathematically such a way that can be pretty close to define the colour range of colour related devices.
There has a whole lot of colour spaces available here. But the most common colour spaces are sRGB, Adobe RGB, ProPhoto RGB and CMYK (for printers).
Remember RGB is not a colour spaces. It is a Digital colour model.
RGB Group of Colour Space:
All colour spaces in RGB colour model use three axis for Red(r), Green(g) and Blue(b).
Here you can see At the point 'O' where r, g and b values are 0 that is pure Black. At point 'P', where r,g and b values are maximum that is pure White. Throughout the 'OP' diagonal all colours are grayscale.
The maximum value depends on bit number we will take about it later.
sRGB:
sRGB was developed by both HP and Microsoft in 1977, where "s" can be interpreted as "standard".
Now a days almost 99% of devices use sRGB colour space like computers monitors, TV screens, mobile phones, tablets etc. Some digital camera has sRGB colour profile set by default.
Web only use sRGB colour space. So, if you want to upload any image in web convert it to sRGB colour space to get more accurate colour. Otherwise web will convert it into sRGB and the algorithm of conversation is not as good as Photoshop.
This colour space has pretty small range compared to what our eye can see.
Adobe RGB:
Adobe RGB colour space was developed by Adobe systems in 1998. It was designed to encompass most of the colours achievale on CMYK colour printer but by using RGB primary colours. Some photographic monitor use Adobe RGB colour space. Digital cameras have Adobe RGB colour profile in build.
This colour space is bigger than sRGB colour space. That means it contains more saturated colours (Blue and Magenta has almost equal saturation).
If Adobe RGB is choosen in DSLR then it can capture more colour information than sRGB.
In photography it is recommended to work with Adobe RGB.
ProPhoto RGB:
ProPhoto RGB colour space, also known as ROMM RGB (Reference Output Medium Matric) was developed by Kodak. This colour space encompass over 90% of visual colours.
This colour space is much bigger than Adobe RGB colour space. Due to that big range of colours it has more saturated colours than Adobe RGB colour space. One of the downside of this colour space is approximately 13% of representative colours are imaginary.
When working in colour spaces with such a large gamut, it is recommended to work in 16-bit colour depth to avoid posterization effects. This will occur more frequently in 8-bit modes as the gradient steps are much larger.
CMYK Colour Space:
CMYK colour space is a colour space only used for printers. CMYK colour space works in the opposite way to the additive RGB grouped colour space. In CYMK colour space the process is subtractive and the inks subtract brightness from White (Paper used for print) by layering Cyan, Magenta, Yellow and Black to achive a certain hue.
This colour space is pretty small compare to Adobe RGB or ProPhoto RGB colour spaces. sRGB colour space contais a big area of it.