The technical terms that come up most often, explained briefly, so that a quote or a conversation with an operator does not stay incomprehensible.
VPS
Virtual Private Server: an isolated virtual machine created inside a physical server, with its own operating system and guaranteed resources.
KVM
The virtualisation technology built into Linux. It gives strong isolation between instances, which is why a KVM-based VPS is worth choosing over older, more weakly separated solutions.
vCPU
A processor core allocated to the virtual machine. It matters whether the capacity is guaranteed or merely shared; the difference shows at peak times.
LTS
Long Term Support: an Ubuntu release supported for the long run, with five years of standard security fixes. A live server always calls for LTS, never an interim release.
Hardening
Security tightening: switching off unnecessary services, narrowing permissions, restricting the ways in. It is part of the installation, not an afterthought.
SSH key
Login with a cryptographic key pair instead of a password. It cannot be guessed or brute forced, which is why we switch password SSH login off.
Firewall (UFW)
It only lets in the ports that are genuinely needed. Everything is denied by default and only the necessary services get an exception.
Fail2Ban
Watching the logs, it automatically bans IP addresses that repeatedly fail to log in. That is what puts an end to the bot attacks flooding the logs.
Reverse proxy
A server placed between the visitor and the application (typically Nginx) that handles encryption, compression and traffic distribution.
Varnish
A full page cache in front of the web server. Most visitors get a finished, stored page, so PHP and the database are never troubled.
Redis and Valkey
In-memory caches for sessions and object caching. Valkey is the open-source branch that carries Redis on.
PHP-FPM pool
An isolated group of PHP processes. A separate pool per website means that one site being overloaded or compromised does not take the others with it.
OPcache
It keeps the precompiled PHP code in memory, so the source does not have to be parsed again on every request. Free speed, when it is sized properly.
Cron
The engine of scheduled background tasks: indexing, sending e-mail, generating feeds, backups. If cron does not run, the system appears to work while a great deal quietly fails to happen.
systemd
The service manager of Ubuntu. It starts and keeps background processes alive, and restarts them if they stop unexpectedly.
RPO
Recovery Point Objective: how much data we are willing to lose in the worst case. With one backup a day that is at most a day of work.
RTO
Recovery Time Objective: how long the system takes to come back after a serious failure. With a lot of data that is measured in hours, and it is worth knowing in advance.
Snapshot
A point-in-time image of the whole virtual machine. Good for a quick rollback, but no substitute for versioned backups on separate storage.
SPF, DKIM, DMARC
Three DNS-based e-mail authentication standards. Together they make sure that mail sent in your name counts as genuine and does not end up in spam.
TTL
The lifetime of a DNS record. Lowered before a server migration, the switchover goes through in minutes rather than hours.
Uptime
The proportion of time a service is available. 99.9 percent means about 43 minutes of outage a month; it is worth thinking in numbers rather than adjectives.
Load average
The indicator of system load. On its own it says nothing; it makes sense relative to the core count, and that is when it becomes clear whether capacity is tight.
Staging
A copy of the live system where updates and development work can be tried out first. Whoever tests directly in production is testing with their customers.
WAF
A web application firewall that filters incoming requests for known attack patterns. Good additional protection, but no replacement for updates.