Virtualization and resources
Memory (RAM)
Memory for running programs. Too little memory — the server slows down or crashes.
RAM holds what programs are working with right now: web server processes, the database cache, the game world. When memory runs out, the system starts using the disk — and everything slows down many times over.
A WordPress site usually needs 1–2 GB, a shop or 1C-Bitrix 4 GB and up, databases and game servers — the more the better. The VPS calculator helps pick the amount.