Time to live (TTL) is what dictates how long your records stay cached. For example, for how long your A record will be cached before the retreival of a new copy of the record from DNS servers. The record storage is known as the DNS cache, and the act of storing records is called caching.
When a caching (recursive) nameserver queries the authoritative nameserver for a resource record, it will cache that record for the time (in seconds) specified by the TTL. If a stub resolver queries the caching nameserver for the same record before the TTL has expired, the caching server will simply reply with the already cached resource record rather than retrieve it from the authoritative nameserver again. TTL for NXDOMAIN responses is set from the minimum of the MINIMUM field of the SOA record and the TTL of the SOA itself, and indicates how long a resolver may cache the negative answer.
Shorter TTLs can cause heavier loads on an authoritative nameserver, but can be useful when changing the address of critical services like Web servers or MX records, and therefore are often lowered by the DNS administrator prior to a service being moved, in order to minimize disruptions.
The units used are seconds. An older common TTL value for DNS was 86400 seconds, which is 24 hours. A TTL value of 86400 would mean that, if a DNS record was changed on the authoritative nameserver, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the change.
At ClouDNS, the default TTL is 3600 seconds (1 Hour). The TTL can be set from 60 seconds (1 Minute) to 2592000 seconds (1 Month) for every single record. This option is available only for accounts with Premium/DDoS/GeoDNS subscription. If your account is on Free subscription, you cannot change the TTL.