A picket line is when union members on strike stand outside their workplace holding placards to persuade other people not to go to work, and to tell people why they're striking. This school in Manchester is closed completely.
Workers on strike are angry with Prime Minister David Cameron who's on this poster because it's his government who are making the changes they don't like.
These teachers are striking because they don't want the government to change their pensions. They're standing outside their school on what's called the picket line.
The government says they have to make the changes because there isn't enough money to go around.
A strike is when a group of workers agree to stop working to protest against something they think is unfair. This man is carrying placards to show what he's angry about.
It's thought tens of thousands of pupils are missing lessons as teachers stage the one-day strike.
This T-shirt is worn by a union member who's striking. He blames big banks for the fact there's not much money around in the UK at the moment.
Workers in lots of different jobs are on strike today to protest against the government. These people are putting up tents really early in the morning in Trafalgar Square, London.