The three validation levels, Domain Validation, Organisation Validation and Extended Validation, differ in how much checking the certificate authority does before issuing. They do not differ in encryption strength. A free DV certificate and a several-hundred-dollar EV certificate protect the connection identically.
That single fact resolves most of the confusion, because the marketing around certificates strongly implies otherwise.
Domain Validation
The authority checks you control the domain, usually by having you place a file on the server or a record in DNS. Automated, and issued in minutes.
This is what Let's Encrypt issues, what Ahosting includes free on every plan, and what the overwhelming majority of sites should use.
What it proves: this connection is encrypted and this really is the server for that domain. Nothing about who owns it.
Organisation Validation
Domain control, plus a check that the organisation named exists as a legal entity. A human at the authority verifies business registration details, which takes days rather than minutes.
The organisation name appears in the certificate details, which a visitor sees only if they open the certificate and read it. Very few people ever do.
Extended Validation
The most thorough vetting: legal existence, physical address, operational status, and confirmation the request is authorised.
EV used to display the company name directly in the address bar as a green indicator, and that visible signal was the entire commercial case for it. Browsers removed that treatment, because research showed users did not notice it and did not change behaviour when it was absent.
Today an EV certificate looks the same as a free one in the address bar. The verification still happened; nobody can see that it did without digging.
The other axis: how many names are covered
Separate from validation level, certificates differ in scope.
Single domain covers one name, usually with www included as a courtesy.
Wildcard covers a domain and all its direct subdomains: *.example.com handles blog, shop, staging and anything else you add later without reissuing.
Multi-domain covers several unrelated domains in one certificate. Convenient to manage, and it means every domain listed is visible to anyone inspecting the certificate on any of them, occasionally an issue if the sites should not be publicly linked.
Wildcards genuinely save work if you create subdomains often. Note that a wildcard covers one level only: *.example.com does not cover a.b.example.com.
Which to choose
Almost every site: DV, free. Blogs, business sites, portfolios, small shops. The encryption is identical and it renews itself, which removes the most common cause of certificate failure: somebody forgetting.
Consider OV or EV when a partner, payment processor or auditor requires it in writing, or when the warranty attached matters to your risk assessment. Both are real reasons; neither is about better security.
Do not buy one because a page says visitors will trust you more. Visitors cannot see the difference. The claim was arguable when EV had its own address bar treatment; it is not arguable now.
Warranties
Paid certificates advertise a warranty: sometimes over a million dollars. It sounds like the main difference between free and paid, and it is worth understanding what it covers.
It protects the end user, not you, and only against losses caused by the authority mis-issuing a certificate. It is not insurance for your site being compromised, and claims are rare enough that it should not drive the decision.
Free certificates and short lifetimes
The main practical objection to free certificates is that they expire quickly: 90 days in place of a year.
That is a strength, not a weakness, because renewal is automated. A certificate renewing every 60 days without human involvement fails far less often than an annual one that depends on somebody remembering a date twelve months out.
Expired certificates are one of the most common causes of a site becoming unreachable, and almost all of them are annual certificates nobody renewed. Renewing your SSL certificate deals with both paths.
What actually matters more than the type
Whichever certificate you have, the same three things decide whether it does its job.
It must cover every name the site is reached by, including www and any subdomain. It must not expire. And the whole site must actually use it: HTTP redirected, no mixed content, no login page secured while everything else runs plain.
A free DV certificate correctly deployed protects your visitors better than an EV certificate on a site that still serves half its pages over HTTP. Forcing HTTPS and fixing mixed content cover the two steps people skip.
Whichever type you buy, it starts with a key pair and a signing request. How to Generate a CSR and Manage Private Keys sets out generating them properly.
Certificates can also identify the visitor in place of the server, which is a different tool for a narrow job. How to Use Client Certificates to Restrict Access has the detail.