At the time of writing, some properties of the UK one penny coin are as follows:
Substance: copper-plated steel
Diameter: 20.32 mm
Width: 1.65 mm
Mass: 3.56 g
Value: 0.01 pounds sterling
Commonly called a 'penny' or sometimes 'a one P coin'.
Even though one penny and two penny coins are referred to collectively as 'coppers', they haven't been made of copper since 1860.
Pennies from before 1992 (and in fact, one proof set minted in 1999), were made of bronze. The metal was changed to copper-plated steel to reduce production costs. Both versions of the penny have the same mass, but the bronze coins had a higher density, and so they were slightly thinner.
When the new steel coins were issued, there was some concern that they could potentially become magnetised. If people kept them in their pocket or purse along with their credit cards, it was theorised that the magnetic stripes on the cards could be damaged by the magnetism. In practice this does not appear to have been a problem.
The old bronze coins tended to quickly become dull in appearance. The newer copper-plated coins stay shinier for much longer. However, now that the new coins have been in circulation for a while, the copper coating on some of the older ones has started to rub off, producing coins that look silver in colour. Some coin dealers report excited callers hoping to sell the 'rare solid-silver one penny coin' they have 'discovered'.