In the UK, most lamp post lights are an orangey colour. This is because they contain hot, gaseous sodium, which has been excited by having an electric current passed through it. The electrons in the sodium atoms are excited to a higher energy level than they were at before, but they cannot remain there as it is unstable. They then fall down to a lower energy level, and emit the excess energy as a photon, in this case of an orange colour.

Looking at the spectrum of a street light, you would see that for most of the spectrum no light is emitted, but that there are two sharp lines quite close to each other which are both orange. These are known as Sodium D lines, and they are emission lines.

If, on the other hand, you had a container filled with hydrogen gas and you shone a white light through it, and then looked at the spectrum of light on the other side, you would see that it was mainly continuous like a rainbow, but there are a few dark lines - absorption lines. These occur because the electrons in the hydrogen gas have absorbed photons of the correct energy to move them to a higher level. When they drop back down, however, they emit a photon in a random direction, not necessarily the direction that the incident photon was travelling in to begin with.

All different elements and molecules have different absorption lines, and by observing spectra from distant stars and galaxies, it is possible to work out which substances are present.

A well known set of lines in the visible spectrum are the Balmer lines of Hydrogen. These are named for a Swiss schoolteacher who analysed them, and occur when electrons in hydrogen move to the second energy level.