A Domain Name is a human language representation of an IP address.
An IP Address is what every computer on the internet uses to address itself to the other computers on the the Internet (using the network protocol called TCP/IP. IP (v4) Addresses look like 188.8.131.52
When someone types in a domain name like www.domain.com, their browser communicates with a series of root domain name server that act as a dictionary and provides the IP address associated with that domain name. Then the browser can use that IP to communicate to the server that the website is hosted on.
TLD (or Top Level Domain) is the last part of a domain name... the com, net, org, two letter county domains, and the several other TLDs out there.
SLD (or Second Level Domain) is the most human readable part of the domain name. In a domain name like www.domain.com, domain is the SLD. An SLD can have any alphanumeric character in it (a-z, 0-9), dash or minus ( - ), and underscore ( _ ). But it cannot have spaces between characters.
Sub-Domain (also called third level domains) are technically called Canonical Domains (or CNAMEs) for short. Sub-Domains are like having an extra domain name and can be almost anything you like. In a domain name like subdomain.domain.com, subdomain is the sub-domain. It works the same as a regular domain name.
A-Records (or Address Record) is the basic and most important DNS record. A-records point to an IP address. Your short domain name (without the www), NS, and FTP should have A-records. Subdomains sometimes have A-Records too. A-records can point to any IP-address.
CNAME-Records (or Canonical Domains) include subdomains and Aliases, CNAMEs are used to point to a domain name or to a file in a domain. However, CNAMEs should always point to an A-record, not another CNAME. It is a common practice to create a CNAME for www and for subdomains that are actually hosted by your domain. CNAMES can also be used as temporary aliases to point your domain to another domain.
Note: when pointing a CNAME, always put a period after the domain (ie: ftp -> CNAME -> domain.com.)
MX-Records (or Mail Exchange) point to the name of an email server and holds a preference number for that server. MX-records must point to an A-record or in some situations an IP-address.
Example DNS Record:
Host Name IP Address/URL Record Type
@ 184.108.40.206 A-Record
www domain.com. CNAME
ftp 220.127.116.11 A-Record
mail 18.104.22.168 A-Record
ns1 22.214.171.124 A-Record
ns2 126.96.36.199 A-Record
subdomain1 domain.com. CNAME
subdomain2 domain.com. CNAME
MX-Record: mail -> domain.com. -| Preference = 10
@ is short domain (domain.com)
Put period after domain name