DNS stands for Domain Name System and is the system to translate friendly hostnames like example.com into the IP addresses of the server the website is hosted on. 

• A Record - An A record is a type of DNS record used for pointing the hostname to the IPV4 address (51.91.204.1).
• AAAA Record - An AAAA record is a type of DNS record used for pointing the hostname to an IPV6 address such as (2a00:1450:4009:809::200e).
• CNAME - canonical for pointing a hostname to another hostname. For example, if you want to have a sub like about.example.com. You always want it linked to the same server so you'd set up a CNAME to point to it so if you ever move then you only need to update the central a record and don't have to worry about all the different ones. For example mail.me to mail.google. Would set up as a CNAME to point to google and your config is set while google can redirect you to their changing IP. 
• NS - nameservers are DNS servers which help to resolve domains. For example at the registry would have ns1.atd. 
• MX records - defines where the email goes for particular domain name. For example, you can have multiple MX records per domain.