From gbhackers.com
Operating Systems can be detected using Ping Command, Ping is a computer network administration software utility, which used to find the Availability of a host on an Internet Protocol (IP) network.
Ping operates by sending Internet Control Message Protocol (ICMP) Echo Request packets to the target host and waiting for an ICMP Echo Reply.
TTL is simply meant, how long a resolver is supposed to cache the DNS query before the query expires and a new one needs to be done.