華人is a term that describes all people that has Chinese heritage,including Taiwanese,Hong Kongniese,some Singaporean and Malaysian,Chinese who immigrated to America,to Europe,to all over the world,but we just use American華人to describe them,which is美籍華裔,etc.One of the most important factor that you can use to determine who is a華人is that they know Chinese,Cantonese,or any other Chinese dialect.However,華人is a very ambiguous term,is more of a culture thing,is more of what people believe where they belong to.漢人means the ethnicity.China has 56 ethnicities,漢takes up more than 90%of the entire Chinese population.漢人doesn’t need to know Chinese language,thus they probably don’t know the term漢人.
However,both of the terms are rarely used except for census.