Double yellow line
A double yellow line is a common road marking meaning different things in many parts of the world.
Double yellow lines along the edge of the carriageway indicate that waiting restrictions apply to the road (which includes the carriageway, pavement and verge). A driver may stop for passengers to board or alight and to load or unload (unless there are also 'loading restrictions' - see below). The regulation applies to all vehicles other than those with disabled parking permits. The restriction applies from the centre of the carriageway to the back of the footway.
Double yellow lines mean no waiting at any time, unless there are signs that specifically indicate seasonal restrictions. They were first introduced in the UK by section 51 of the Road Traffic Act 1960 (repealed in 1972 and replaced by later legislation).
Loading and unloading IS allowed on double yellow lines at any time, unless there are additional markings indicating that there are 'loading restrictions'. Two short yellow stripes at regular intervals across the kerb or edge of the carriageway indicate that loading and unloading is NOT permitted at any time (these are not accompanied by sign plates). A single short yellow stripe at regular intervals across the kerb or edge of the carriageway indicates that loading and unloading is NOT permitted at the times shown on accompanying black and white sign plates.  Loading/unloading time is restricted to either 20 or 40 minutes depending upon the location (borough) if loading is observed. You must not cause an obstruction to traffic or pedestrians. 
Republic of Ireland
Double yellow lines on the side of the road has a similar meaning (No parking at any time) as the United Kingdom.
Hong Kong is one of the regions using double yellow lines on the side of carriageways outside the UK.
A double yellow line, compared to a double white line, indicates a no-passing restriction. Double white is just a no passing suggestion.
A double yellow line means no stopping. This means that unloading of goods and alighting of passengers is not permitted. 
Singapore, an ex-crown colony of the UK, also uses double yellow lines to indicate no parking at the sides.
Mauritius was once a British colony, and today still applies double yellow lines to mark restricted parking areas at all hours on the sides of carriageways.
Australia and Canada
In Australia and Canada, a double yellow line in the centre of the road means that it is unsafe for traffic travelling in either direction to pass. It is usually found on rolling hills or through corners where visibility is limited.
In the province of British Columbia, it is against the law to touch or cross solid double yellow line at any time, even to avoid obstructions on the highway. The only exception to this is when a vehicle is entering or exiting the highway, given a vehicle can safely do so without affecting the flow of other vehicles.
I may read it wrong, that Section 155 contradicts what is written in wikipedia about avoiding obstruction in BC:
Highway lines 155
1) Despite anything in this Part, if a highway is marked with (a) a solid double line, the driver of a vehicle must drive it to the right of the line only, (b) a double line consisting of a broken line and a solid line, (i) the driver of a vehicle proceeding along the highway on the side of the broken line must drive the vehicle to the right of the double line, except when passing an overtaken vehicle, and (ii) the driver of a vehicle proceeding along the highway on the side of the solid line must drive the vehicle to the right of the double line, except only when finishing the passing of an overtaken vehicle, and (c) one single line, broken or solid, the driver of a vehicle must drive the vehicle to the right of the line, except only when passing an overtaken vehicle.
(2) Subsection (1) (b) (i) and (c) do not apply if a driver is avoiding an obstruction on the highway and first ascertains that the movement can be made with safety and without affecting the travel of any other vehicle.
A double yellow line is a painted marking between opposing sides of a highway. It consists of two parallel, solid yellow lines located between the two directions of traffic flow and its presence usually indicates a no-passing restriction or no passing zone. Obvious exceptions to this no-passing restriction include emergency maneuvers or temporary traffic flow changes due to road work. Often the double yellow line has sections where one of the lines becomes dashed (in which case it is no longer a "double yellow"), indicating to the drivers traveling on the side closest to the dashed line that they may pass when it's safe.
In some states, it is not against the law to overtake vehicles in the presence of solid yellow lines if it is safe to do so. For example, Vermont State Law also allows passing across the double yellow line when no traffic is on the opposing side, however, one must pass quickly and return to the proper side.  Pennsylvania also permits passing on double yellow lines when not also posted with "Do Not Pass" signage.  However, this is unusual as most states have a ban on crossing a double yellow line except when turning, or when pedestrians, bicycles, or other obstructions in the road make it necessary. Overtaking another vehicle across a solid yellow line is usually considered a serious traffic violation in most states. 
- https://www.gov.uk/waiting-and-parking/waiting-and-parking-238. Retrieved 2013-04-30. Missing or empty
- "Road markings". direct.gov. Retrieved 2010-04-01.
- "Road Traffic Act 1960 - 51 Traffic signs". Legislation.gov.uk.
- "Parking Meter Zones (Double Yellow Lines)". Hansard. "Mr. Marples: Double yellow lines on the carriageway in a parking meter zone indicate a ban on waiting at all times, except for the picking up or setting down of passengers; they also indicate a ban on loading or unloading between 8.30 a.m. and midnight. I have authorised the use of these carriageway markings under the powers given to me in Section 51 of the Road Traffic Act, 1960."
- londoncouncils website
- "Traffic signs & road markings for parking". Road Safety Authority. Retrieved 16 April 2013.
- "Traffic signs & carriageway markings regulations". Government of Malta. Retrieved 16 April 2013.
- Motor Vehicle Act Section 155 BC Laws
- Motor Vehicle Act Section 156 BC Laws