Formal or literary term for people other than oneself or a specified person. Often used in contrastive or exclusive contexts.
It is a state of mind that others cannot understand.
This work cannot be done by anyone else.
他者 is a formal, often academic term for 'the other' as a concept, whereas 余人 is an old-fashioned way of saying 'other people' in exclusive contexts like 余人をもって代えがたい.
自分 means 'oneself', the natural opposite of 余人, which refers to people other than oneself or a specified person.
Standard kanji form; the reading よにん is rare and not normally used in modern Japanese.
Compound of 余 (yo, 'remaining, other') and 人 (jin, 'person'). The reading よにん is a rare alternative that is not commonly used.