I think it depends how many explanations you need to put in and how long they are or how the information of more explanation needs to be compared to the other information. Plus, you have to factor in how flexible your site is, coding-wise and what your team can accomplish.
Opening the link in a new window is great when you have a few things as it's easy to code. Plus, it's important to have in a new window if there's information the customer needs to compare from the explanation. (Like Rene mentioned, sizing charts for clothing.)
You can also use a hover to show the information, but I'd only use it if the explanation is super short, like say a definition.
The third option is having the page expand with the information when they click on it. Of course, this option will be the most technical to code.