The domestic electricity supply in the UK is called the mains. It is alternating current (a.c.), which means the current keeps reversing direction. This is different from a battery, which gives direct current (d.c.) that flows in one direction only.
The frequency of the mains supply is 50 Hz. The unit of frequency is the hertz (Hz), and 50 Hz means the supply completes 50 cycles every second. So in 20 seconds it completes 1000 cycles.
The potential difference of the mains supply is about 230 V. So the UK domestic supply is a.c. at 50 Hz and about 230 V.