CNAME records are domain name aliases.  On the Internet, machines will often have multiple functions such as web-server, ftp-server, chat-server etc.  CNAME-records can be used to give a single computer multiple aliases to mask this.
For example machine “www.machine.com” may be both a web-server and an ftp-server, so the two CNAME-records are defined as www.machine.com = [...]

YES is the simple answer.  Just go into your DNS management section and add A records for blog and forums putting in the appropriate IP addresses.  You keep the same nameservers – they point to a DNS zone. Here you would need to add the extra A records for blog.domain.com and forum.domain.com  If you activate [...]