The YMCA have a Discriminatory policy in place whereby if your forced to stay in one due to being made homeless you may not apply to work in the building. Even if they have Jobs advertised on the notice board when you go in.
The YMCA promotes itself as being a Christian Charity but it doesn't seem very christian to tell a homeless person who's in dire need of employment they may not have a Job because they're staying in the building!
Discrimination means treating some people differently from others. It isn't always unlawful - after all, people are paid different wages depending on their status and skills. However, there are certain reasons for which your employer can't discriminate against you by law.