there are little differences here and there and they depend on your broker. with penny stocks (anything under $1) you will get charged extra per share of each share you buy; for scottrade its half the price of the stock extra so... $0.50 stock will cost $0.75 per share). margin accounts are only given to accounts that have more than $2000 in the account and have an interest rate to them just like in wss. when buying and selling a stock there is a settlement date in which you have to wait (3days for scottrade) in order for those funds to be usable again. so you cant buy a stock one day, sell it the next, and then the same day buy another stock. you will be considered a daytrader and by market rules a daytrader must have at least $25000 in your account or else you will be flagged and penalized.
stocks do get traded quickly depending on the stock (average volume per day). with stocks like google, walmart, disney, altria, monsanto, etc. prices fluctuate quickly (pretty much on a per second basis). and it is very hard sometimes to get your exact stop/limit order because of this. there is also the spread that you have to worry about (the difference between ask/bid).
shorts work in the opposite way buying a regular stock does. you "borrow" 'x' amount of shares from your broker at the designated price. so 100 shares of stock XYZ at $10. you then hope for the stock to go down and buy to cover your position at a lower price making you a profit; if the stock goes up then you lose money. in order to do this one must be eligible and have a margin account.
im pretty sure i left out A LOT of stuff but thats all that comes to mind. if you have any more specific questions i'd be glad to answer them to the best of my ability