noun
one (flat object); one sheet
Counter for thin, flat items such as paper, tickets, plates, CDs, or clothing. Used as a noun meaning 'one such item'.
紙を一枚ください。
Please give me one sheet of paper.
このCDは一枚五百円です。
This CD is 500 yen each.
General counter for one item; 一枚 is specifically for flat objects.
Counter for one person; 一枚 is for objects, not people.
Compound of 一 (いち, 'one') and the counter 枚 (まい) for flat objects.