Prime Meridian & GMT4.18 - Know that mean time at any point along the Prime Meridian is defined as Greenwich Mean Time (GMT), which is the same as Universal Time (UT)
The Prime Meridian
A meridian is a line of longitude that runs between the poles and the observers position. The Prime Meridian is an internationally agreed imaginary line that runs through the Royal Observatory in Greenwich, London. This line of longitude is from where longitude is measured.
If you stood on this meridian, one half of you would be in the Eastern half of the world and the other in the western half. Just like my feet in the picture opposite, look.
Greenwich Mean Time
GMT (Greenwich Mean Time) is the timezone centered at the prime meridian and is used in the UK, Ireland, Iceland, Portugal and some several more African countries.
Coordinated Universal Time - UTC (sometimes refered to as Universal Time) is not a time zone like GMT but a time 'standard'. In astronomy and space exploration it is espcially useful as when events such as occulatations occur or a lander event on Mars, UTC is used throughout the scientific community as using one timezone is better than having an astronomer in New York using times different to an astronomer in Paris.
Universal Time is based on International Atomic Time which uses an average of hundreds of atomic clocks around the work. In practice the same as GMT but deviates to it by about a second to compensate for the slowing of Earth's rotation and is adjusted at times with additional leap seconds.
Daylight Savings Time
From Time and Date website: "Daylight Saving Time (DST) is the practice of setting the clocks forward 1 hour from standard time during the summer months, and back again in the fall, in order to make better use of natural daylight." They said it better than me. They're American though and use the word 'fall' instead of 'autumn'. Sorry. In Britain we put our clocks forward at the end of March (we call this 'British Summer Time') so we have more light in the summer evenings and put our clocks an hour back to GMT in October so our mornings are less dark.