2) What is the difference between a cash account and a margin account
A cash account allows you to trade stocks with Your Own cash. Margin trading, short selling, option trading and future trading are unavailable in a cash account. There is no pre-established limit on the number or frequency of T+0 trades.
A margin account is a cash account plus it supports margin trading and short selling (leverage up to 400%).And it allows futures trading. There is no pre-established limit on the number or frequency of T+0 trades.