"Sick" refers to a short-term problem like the flu. "Ill" refers to a more general and longer term problem with health, which can be mentally and/or physically.

There is a difference here between British and American English - at least for the moment. The first two answers have given you the distinction as the words are used in American English. However, as Ruthi says, some speakers of US English seems to use the terms more or less interchangeably. In British English, though, the words have two different meanings. 'Ill' means 'not well'. Meanwhile, the word 'sick' refers to a feeling of nausea and a need to vomit. If a speaker of British English says 'I feel sick', it means that they feel nauseous. For example, if you eat six cream cakes then go on a fairground ride, you will feel sick. If a British person says 'I was sick', we would probably understand this to mean 'I vomited'. That said, British English does use 'sick' in certain situations to refer to a short-term period of being unwell. For example, you can take a 'sick day' off work, 'call in sick' to say that you won't be in work today, and get a 'sick note' from the doctor. Like Ruthi, I grew up learning that 'ill' meant 'not well'. If I you have a temperature, a fever, an infection or spots all over you, this means that you are ill - you are not sick. Sick means 'throwing up'. Similarly, I only came across the word 'sick' meaning generally unwell when I heard Americans using it in this sense. However, the differences between British and American English are becoming ever smaller. The younger generation of British people are beginning to use 'sick' and 'ill' almost indiscriminately, as Americans do. Give it a few years, and there'll be one global English.

I agree with the previous answers,but I have noticed that some Americans use these terms interchangeably or use sick more than ill.I never used to use "sick" before I met American speakers and I found sometimes they didn't understand the word "ill" so I began to say "sick" as well.

Sick is less formal than ill. And usually sick describes short-term disease while ill describes both long and short-term disease.