An attempt at a simple answer for the normal investor:
A normal investor buys stock then later sells that stock. (This is known as "going long", as opposed to "going short"). For the normal investor, a stop order (of either kind) is only used when selling.
A stop-loss sell order (or stop sell) is used to sell your stock when it has fallen too much in price, and you don't want to suffer more losses. If the stock is at $50, you could enter a stop sell at $40, which means if the stock ever falls to $40 or lower, your stock will be sold at whatever price is available (e.g. $35).
A stop-loss limit sell order (or stop limit sell) is the same, except you are also saying "but don't sell for less than my limit price". So you can enter a stop limit sell at $40 with a limit of $39, meaning that if the stock falls to $40, you will then have a limit order in effect to sell the stock at $39 or higher. Thus your stock will never be sold at $35 or any value below $39, but of course, if the stock falls fast from $40 to $35, your limit sell at $39 will not be done and you will be left still owning the stock (worth at that moment $35, say).