In the UK, most streetlights are an orangy 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 streetlight, 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 emmission lines.

If on the other hand you had a container 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 necesarrily the direction that the incident photon was
travelling in to begin with.

All different elements and molecules have different absoption 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.