Just a quick definition, a foundation is a wooden or metal frame with a sturdy, flat board attached to the top. This is to be placed on the bed frame between the headboard and the footboard or used on its own, to give your mattress even support. It is not the same thing as a box spring. Ultimately, the best support system for your bed will depend on the mattress type you use.
For all-foam beds, particularly memory foam, a box spring is a big no-no. A traditional box spring can shorten your foam mattress’s lifespan by damaging its support and consistency. Using a box spring can even void the warranty on your mattress. Instead consider adjustable bed frames, foundations, and platform beds for a foam mattress.
Hybrid and innerspring mattresses allow for a bit more flexibility, typically pairing well with either a box spring or a foundation. They also pair well with most bed frames.
Much like you would for a mattress, check what the listed weight capacity for a foundation is as well, to be sure it can provide the support needed. Check also its trial period and its warranty. Many have limited warranties that last between three to 10 years.