Hosting has a vocabulary that everyone uses and nobody defines, and most of the confusion in support conversations comes from two people using the same word for different things. This is what the common terms actually mean, grouped by where you meet them.
The three things you are buying
Domain. The name, registered for a period and renewed. It is not the website and it is not the hosting; it can point at any host, and it is the one whose loss is hardest to reverse.
Hosting. The space and processing that serves your site.
DNS; the directory connecting the two. Changing hosts is a DNS change; changing registrars is not.
They renew separately and can be bought from different companies, which is why "my hosting expired" and "my domain expired" are different emergencies. For the second, see What Happens When a Domain Expires.
The hosting tiers
Shared hosting. Many accounts on one server, sharing its resources. Cheap, managed for you, and limited by what your neighbours are doing.
VPS. A virtual machine with resources reserved for you. More control and more responsibility.
Dedicated server. A whole physical machine.
Reseller hosting; an account you subdivide and sell on, with WHM to manage the accounts under it.
Managed or unmanaged, whether updates, security and recovery are theirs or yours. This affects the real cost more than the specification does. How to Compare Shared, VPS and Dedicated Hosting sets out choosing.
Control panels
cPanel, where you manage one hosting account: files, email, databases, domains.
WHM, where you manage accounts: creating them, setting packages, server-level settings. A reseller has WHM; an end customer usually has only cPanel.
File Manager, phpMyAdmin, Terminal: tools inside cPanel for files, databases and a command line respectively.
Domains and the names under them
Subdomain. A name under your domain, like shop.example.com. Free, and it is part of your domain.
Addon domain. A separate domain hosted in the same account, with its own directory.
Parked domain or alias. A second domain showing the same site. The right tool for owning several spellings of your name.
Nameservers. The servers that answer DNS questions about your domain. Set at the registrar; this is what "pointing a domain" usually means.
DNS record types, briefly
A and AAAA; the address of a server, for IPv4 and IPv6.
CNAME. An alias pointing one name at another.
MX, who accepts mail for the domain.
TXT: arbitrary text, used for verification, SPF, DKIM and DMARC.
TTL: how long others may cache the answer, which decides how quickly a change takes effect. Which DNS Record Type Do You Need explains choosing between them.
Certificates
SSL/TLS certificate: what makes HTTPS work. TLS is the current name; SSL is the one everyone still says.
AutoSSL, automatic issuance and renewal for domains on the server.
CSR and private key. The request you send to buy a certificate, and the secret half that stays on your server.
Wildcard: covers all subdomains at one level, and notably not the bare domain unless it is added. There is more on the shapes in Wildcard and Multi-Domain (SAN) Certificates.
Resources and limits
Disk, files, databases and mail together. Mail is usually the part that grows unnoticed.
Bandwidth or transfer, data moved per month. Port speed is how fast at any instant. They are different limits with the same nickname.
Entry processes: how many PHP requests may run at once. This is the limit a busy site actually hits, and exceeding it makes requests queue rather than fail, which is why the site hangs and then loads.
Inodes. The number of files, counted separately from their size. Thousands of tiny cache files can exhaust it while the disk looks empty.
Mail terms
IMAP keeps mail on the server; POP downloads it to one device.
SMTP is sending, as opposed to reading.
Forwarder redirects an address elsewhere without a mailbox. Catch-all accepts every address at the domain, including the guessed ones spam senders try.
SPF, DKIM, DMARC, DNS records that let receiving servers verify your mail is genuinely yours.
Words used loosely in marketing
Unlimited: there is always a fair-use clause. The useful question is what happens when you exceed it.
Unmetered: usually no monthly cap, with the port speed as the real limit.
99.9% uptime: about 43 minutes of downtime a month, and the exclusions matter more than the number. Understanding Uptime Guarantees and What They Cover goes into reading them.
Free migration: worth asking what is included: sites only, or mail and DNS as well.
Terms that mean different things by provider
Several words in hosting have no agreed definition, and comparing offers on them produces conclusions that do not hold.
Backups can mean daily copies kept for a month, or a snapshot taken for the provider's own recovery that you cannot access. Both are described the same way.
Support can mean a person within an hour or a form with a reply in three days. Managed can mean the operating system is maintained, or only that the hardware is.
Ask what each includes specifically rather than comparing the words. The answer is usually clear when requested and never stated in the description. What to ask before moving to a new host covers the questions.
The units that are easy to misread
Two measurements are routinely confused, and both affect what you are buying.
Storage is quoted in one of two conventions that differ by around seven per cent, which is why a disk sold at one figure reports a smaller number once formatted. That difference is normal rather than a shortfall.
df -h / | tail -1 df -H / | tail -1
Network speed is quoted in bits and file sizes in bytes, a factor of eight. A connection described at one figure transfers files at an eighth of that number, before any overhead.
Neither is a trick, and both produce a sense of having received less than was promised when the convention is not understood.
What a limit does when you reach it
The number matters less than the behaviour at the ceiling, and that is rarely stated.
Exceeding storage may stop writes, stop mail, or trigger a charge. Exceeding transfer may slow the site, suspend it, or bill for the excess. Exceeding a process limit usually produces errors for visitors rather than any notice to you.
quota -s 2>/dev/null df -h ~ | tail -1
Ask which of those applies before choosing a plan on its numbers. A generous limit with immediate suspension is worse than a smaller one with a warning and a grace period.