Photocells (Photo-electric cells) are the sensors (usually situated on top of a lantern) that can measure light levels. When the light level becomes too low, the photocell switches the street light on. Photocells can either be electronic or thermal, or both.
On larger roads, (eg Motorways), photocells are not put on top of lanterns. Instead, there is just one photocell that controls a lot of lanterns. They are situated on embankments and are sometimes seen with a lantern or not.
Originally, before photocells were invented, a small device called a time switch was used. It added/took away an hour every day as the seasons changed, and thus was able to control when the light should be on or off. The only problem arose when there had just been a power-cut and the time switch had not been receiving power. It would lose all of its memory and would allow the lamp to shine all the time. Eventually, time switches were fitted with a battery-back-up system to try and stop this from happening.
As more lanterns were fitted with photocell sockets, the need for time switches decreased. Photocells could just be plugged straight in, whereas with time switches, a lot more programming was required.
Another very old and basic way of switching a lamp on was the conventional hand switch (usually located between the top of the column and the bracket). Many places have had these removed as many hand switches had no form of locking the switch in an on or off position, meaning that anyone could turn the lamp on or off during the day or night. Some are even said to have slipped and made the lamp appear to 'automatically' switch on or off at peculiar times. It could have given passers by quite a shock!