Compute
RAM
Random-access memory holds the data your server is actively working with, and running out forces slow disk swapping or crashes.
RAM is your server's working memory, where the operating system, your application, and its active data all live while running. Run low and the system starts swapping to disk, which is orders of magnitude slower, or the kernel starts killing processes outright. Either way, users notice.
This is the spec where the value gap between hosts is starkest. Hetzner is famous for it: you routinely get two to four times the RAM per dollar compared with the American providers, which is why cost-conscious teams comfortable with Linux flock there. Contabo pushes even more RAM per dollar, with slower support as the trade.
How much you need depends entirely on the workload. A static site sips memory. A database, a JVM app, or a WordPress site with heavy plugins can eat gigabytes. Watch your actual usage before sizing up, because RAM you never touch is the easiest overspend to make.