The reasons for logging your amateur activity fall into three categories: legal, operational and personal.
Legally, Under the terms of your licence you are required to keep a log of your transmissions. This would be invaluable in proving your innocence in an interference complaint.
Operationally, Having a log of past contacts is a resource when filling out that DX QSL card that may have taken months to arrive.
Personally, A log is like a personal radio history reminding you of the people and places you’ve talked to, the nets you participated in and contests you worked.
A DX Cluster is in essence a "chatroom" into which amateur DX hunters can post information about other ham radio stations either worked or heard. Physically, it is a central computer that collects, stores and disseminates information that hams send to it. There are thousands of computers around the world, connected together via the internet.
The networked nature of DX clusters is perhaps its most powerful feature as it gives hams almost instant access to information about rare and unusual DX conditions such as meteor scatter or VHF ducting.
Cluster users will use computer software (for example, many logging programs have this capability) that is capable of communicating with the cluster.
On initial login, users will be asked to provide station information. This allows other hams compare DX openings to their physical position, and hence to make informed decisions about antenna direction for example. New information is passed on to logged in users continuously.
Coordinated Universal Time (UTC) is the standard time system of the world. It is the standard by which the world regulates clocks and time.
The standard before was Greenwich Mean Time (GMT). UTC and GMT are almost the same. In fact, there is no practical difference which would be noticed by ordinary people.
Some websites, like Wikipedia, use UTC because it does not make any country look more important than the others. It offers one time for all the internet (the same time can be used by people all over the world).
Timezones are often named by how many hours they are different from UTC time. For example, UTC -5 (United States east coast) is 5 hours behind UTC.
Note that UTC uses the 24-hour clock. That means there is no 'AM' or 'PM'. For example, 4:00PM would be 16:00 or 1600.