A little background reading seems to be in order, eh? Maybe start here: https://en.wikipedia.org/wiki/Segmental_duplication

The high-level summary is that all of these refer to regions of the genome that have exactly or nearly the same sequence. Large swaths of our genome are made up of duplicated sequence - things like Alus, SINE, and LINEs, just to name a few.

These make many tasks, like mapping short reads, and assembly difficult, because it's hard to determine which of the many possible regions your particular read came from.

RepeatMasker has answers to some of your questions in its FAQ section: http://www.repeatmasker.org/faq.html

Segmental Duplications - Large blocks of 10-300 kilobases which are that have been copied to another region of the genome.

These terms can be difficult to search for with Google. For the strictest definitions I recommend looking up terms in the Sequence Ontology:

For example Tandem Duplication