The four types of domains are as follows:
Top-level domains: Top-level domains (TLDs) refer to the last part of your domain name. e.g. in www.google.com , .com is a top level domain.
Country code top level domain: Abbreviated as ccTLDs, the country code top level domains are restricted for use in certain countries. E.g. .uk can only be used in the UK, whereas .za is the ccTLD for South Africa.
Second level domain: Second level domains, or SLDs, are an integral part of your domain name. The one that gives your brand its identity. E.g. in www.google.com, google is the second level domain. There are also country code SLDs, which can only be used in certain countries. E.g. in www.hostafrica.co.za, .co.za is a country code SLD.
Third level domains: These are a part of the domain, which precedes the second level domain. The www in www.hostafrica.co.za is a third level domain.