The domestic supply in the UK is alternating current (a.c.). This is the supply that comes from the wall sockets in a home. Its two key values are a frequency of 50 Hz and a potential difference of about 230 V.
A frequency of 50 Hz means there are 50 complete cycles every second. One cycle therefore lasts 1 ÷ 50 = 0.02 s. In each cycle the direction of the current reverses twice, so the current changes direction 100 times every second.
A potential difference of about 230 V is much larger than the 1.5 V of a single cell or the 9 V of a small battery. This is why the mains supply is dangerous and must be used with care.