The word über comes from the German language. During the late 1990's, über became a hip, trendy synonym for super; i.e. übercool - supercool.
Uses in German
In German, über is used as a prefix as well as a separate word. In the latter case, it may be a preposition or an adverb depending on context. Eg. sprechen über - speak about, über die Brücke - over the bridge, übernehmen - take over (nehmen = take).
Über also translates to superior. The actual translation depends on context; superior frequently is a correct translation from German when über is a prefix in the German text. One example would be the term Übermensch, which translates to superior human or superman. It is assumed that it is via this translation that the word über entered the English vocabulary (cf. calque).
Non-German Uses
Online Gaming
"Uber" (also "über" or "00ber") is an online gaming term for a player or object that is considered superior. For example, a player who has accumulated the most points or rare and advanced weapons, tools, clothing, or spells available in the online game could be referred to as uber. Rare or powerful items themselves are also frequently called uber. Something does not even have to be a real object to be considered uber, for example a player could use an uber strategy. See also leet.
Conversation
Uber has become a widely used term associated with force. "Uber Glomp" and "Uber Heavy" are the best examples for this.