Ill and sick are both used for saying that someone has a disease or some other problem with their health.

1. You can use either ill or sick after a linking verb.
Ex -
Manjit is ill and can't come to school.
Your uncle is very sick.

2. We You usually use sick, rather than `ill', in front of a noun.
Ex - She was at home looking after her sick baby.

3. We often use ill in front of a noun when you are also using an adverb such as seriously, chronically, or terminally.
Ex - This ward is for terminally ill patients.