Let us consider two scenarios, one where you'd like to reuse the hard drive and one where you don't.
This link shows multiple ways to effectively destroy a hard disk ( Some of these are fun, do take a video)
The other scenario is when you'd like to reuse the hard drive. Here's the basic explanation of data storage -
Any data you store is stored in the form of bits along with meta-data that tell the OS what the data type is. Also there will be a directory which will tell exactly where on the hard disk is the particular data saved. When you delete something, the directory is rewritten to the recycle bin. The data isn't altered. This data can be easily recovered. When you permanently delete something the data still isn't changed, instead the OS is ready to overwrite the data. If the deleted data was not overwritten, it can still be recovered. Even most quick formattings do the same, They just 'free up space' by erasing directories. To keep your data non recoverable, multiple programs exists. What these do is, they overwrite the data over the entire disk randomly a few times. These make it near impossible to recover data. Although a good way is to reuse the disk yourself and fill it up with games & movies a couple of times over the course of 2-3 months and repeat twice. You should be good to go. A final approach is to reinstall an OS on it, this usually clears up the disk properly. A couple of installs and you should be good to go.