The majority of the sites I’ve produced have been on shared hosting plans. In short, a shared hosting plan is where multiple sites are stored on the same server. Every site on that server shares the resource capacity (disk space, bandwidth, etc.) of that server; there’s no cap for how much resource a single account can use. So if a single site on the server consumes of a ton of memory, or gets an insane amount of traffic, the performance of every site on that server will suffer.
Because of their affordability, shared hosting plans are by far the most popular. The price makes shared hosting tempting, but we have a few reasons you may want to consider upgrading to a VPS.