RGB - red, green, blue, this is the standard colour chart used on the web. Then from this also follows on more specific colour ranges guaranteed for the web which is web colours but that can be addressed another time. On the web, what this means is that every pixel on screen is made up of one of these 3 colours. RGB are known as additive colours, when combined you get white light, which produces white
CMYK - cyan, magenta, yellow and black. This is the increased colour range used for print. This allows for more range and detail in the printed colour tones. CMY are subtractive tones, when they are combines they absorb the white light from a page and produce black - so sometimes, when converting RBG one problem you may encounter is that it will use CMY to produce black instead, which is only a very dark black so pure black is added and this is the K element.