Both are used when it is not important to say the exact number of people or identify them exactly.
Someone is used in positive sentences, or in questions where we expect, or want to hear the answer as, yes.
Anyone is used in negative sentences or questions.
- There is someone at the door.
(positive sentence) - Can someone please help me?
(We want to hear yes.) - You never help anyone!
(negative sentence) - Did anybody phone while I was out?
(question)
