In general, the less debt you have, the better. In fact, it's probably good to assume that all debt is technicaly "bad" and harm. That being said, there are some cases where debt can benefit you financially.

Your primary example of beneficial debt is a mortgage used to pay for a home. Not only does it provide you a place to live while you build your equity but it also allows you to maintain ownership of the type of asset that has been historically likely to continuously grow in value. Real estate is, in general, a great investment and will most likely be your biggest in life. Luckily, it's usually your most profitable as well.

Another example of good debt is a student loan. While student loans may be expensive overall, they generally lead to higher-paying jobs and better career paths. So while you may spend a good deal up front to get a better education, the idea is that it will pay you back sevenfold later on. Not only that, but it will help you to get into the aforementioned mortgage.

Ever shopped for Christmas in August to take advantage of bargain prices? Well, good for you. Shopping out of season will save you money in the long-run. This is called "opportunistic debt" and is good only in the event that you end up actually keeping more money than you would have spent otherwise, but you get the point.

Lastly, there's always the credit-building type of debt. If you have expenses you are paying for out of your checking account on a monthly basis, you might as well use your credit card to pay them and use the money you would've otherwise spent on those expenses and use them to pay off your credit card each month. As long as you pay the balance off in full each and every month, you're doing yourself a huge favor. Creditors will smile on you for being trustworthy and if you have a decent credit card, you can earn points or cash-back for money you're spending anyway. And, of course, building your credit will help you to get that most beneficial asset - your home.

Now of course, some debt is unavoidable -- fixing a leaky roof, car repairs, etc., and this is 'necessary debt.' But what you should be careful to watch out for is 'bad debt.' Essentially, you attain bad debt by spending beyond your standard of living or, put simply, having debt that exceeds your ability to pay it off in full each month. Avoid this type of debt at all costs as it will make your credit record look worse to those that would otherwise give you decent loans and lower interest rates.