It depends on whether you want to delegate hosting the subdomain off to a different DNS server (or to the same server, but in a different zone file). You delegate a zone when you want some other entity to control it, such as a different IT department or organization.
If you do, then you need NS records. If not, A or CNAME records will suffice.
Let’s say you have the domain example.com. You have an A record for www.example.com and you want to create the subdomain info.example.com with www.info.example.com as a host in it.
Delegation
In the this situation, let’s further say you have two DNS servers that will be hosting that subdomain. (They could be the same servers that are currently hosting example.com.) In this case, you will create two NS entries in the example.com zone file:
info IN NS 192.168.2.2
info IN NS 192.168.2.3
On those two servers, you will create the info.example.com zone and populate it as you would any other domain.
www IN A 192.168.2.6
No delegation
Here, just add an A record in the example.com zone file, using a dot to indicate that you want to create the www.info host in the example.com domain:
www.info IN A 192.168.2.6
Using CNAME
The decision of whether to use a CNAME is independent of the delegation choice. I generally like to use a CNAME for the “generic” names which point to specific machine names. For example, I might name my machines using an organizational naming convention such as cartoon characters (daffy, elmer, mickey, etc.) or something bureaucratic (sc01p6-serv) and point the generic names to them. If the IP address of the machine ever changes, I need look in only one place to modify it.
www IN CNAME sc01p6-serv
mail IN CNAME sc01p6-serv
sc01p6-serv IN A 192.168.2.6