Single or double yellow lines painted at the edge of the road indicate that a parking ban is in force in that area. The days and times when parking is prohibited is shown on a yellow 'no parking' sign at the side of the road. In some areas, called Controlled Parking Zones (CPZs), signs are not present on all yellow lines. In these areas parking is banned during the times shown on the Zone Entry Sign. Red lines also indicate a parking ban, a sign by the road will tell you at what times parking is banned. If in doubt do not park on a yellow or red line.You may stop briefly on a yellow line to drop off passengers. On some yellow lines you may also stop briefly to load or unload goods - see the Parking Committee for London's Loading and Unloading leaflet for details of the rules concerning loading of goods.
You may not stop on a red line to unload at all, nor to drop off passengers (unless they are disabled).
If you want to know more, read the article, How to Avoid Parking Tickets at: http://www.open.gov.uk/park/avoid.htm#Lines