Zalgo is a combination of 3 different character sets, Combining Diacritical Marks (U+0300 - U+036F), Greek and Coptic (U+0370 - U+3FF) and Cyrillic (U+0400 - U+04FF). It is also known as a "meme" on sites like http://www.4chan.org/ but is also argued to be a "curroption instead. Some websites and computers cannot read these characters even if they have UTF-8 enabled. If you can't see it then try going to "View > Character encoding / encoding > UTF-8". This can affect how fast sites load and how they look.
Encyclopedia Dramatica: "The Zalgo outbreak has since been brought to greater attention by b3ta.com user Evilscary, who created/discovered most of the Garfield and Ctrl-Alt-Del comics featuring Z҉A҉L҉G҉O. Little information has been gathered so far. All that seek the true origin of Zalgo have experienced spontaneous computer errors and power outages."