Immediate or cancel
IOC, as it's usually referred, to as is pretty easy to understand. Simply put when a trader uses IOC on a buy order he is telling his broker to buy all or part of his now. Basically the trader will take the shares anyway he can get them as long as he gets them now. If no stock can be found immediately the order is canceled. The same is true for IOC sell orders (i.e. if no buyers can be found).
Use to Investors
If an investor needs to move ultra quickly than an IOC might be useful (for instance if you thought your stock was going to zero or if you want to pre-empt other buyers based a news story you've read). This condition likely has more appeal to daytraders than long term investors. Also this style of order is more likely to be useful for buys as the investor who uses this for a sell might end up with a partial position of stock.