If you do not sell them on the same day, then you will have to take "delivery" of those shares. The shares will be delivered in your demat account 2 days after you have purchased. So if you do not sell it in the same day, you cannot sell it until 2 more days.
Example:
If bought on monday, you can sell the same quantity on Monday(intraday). But if you choose not to sell on Monday(i.e. take the delivery), then you cannot sell on Tuesday or Wednesday. The shares will be delivered to your demat after Wednesday closing. Thursday onwards you can sell.
There are no differences in between buying for intraday or general buying, though your broker may require you to indicate whether you want to do intraday or delivery while placing the order.