Here is my answer:
I go by many a name,
# has many names: number sign, pound sign, hash, octothorp, tictactoe, etc.
Including one that's quite a shame.
Sometimes people refer to # as the sharp symbol (♯), even though they are completely different. Shame on Microsoft for referring to C# as "C Sharp".
I can take you to an exact location,
In HTML, you can mark a specific location on a web page (let's call it webpage.html) by using an anchor: <a name="anchor_name"></a>. Then someone can go to that exact location on the page by using # and the anchor name (e.g.
http://.../webpage.html#anchor_name). The browser will load the page and automatically scroll the page so that the anchor is at the top of the browser window.
But sometimes only to a nearby destination.
If the anchor is defined near the bottom of the web page, the browser may not be able to scroll far enough to place the anchor at the top of the window. But the anchor will still be somewhere in the window.
If great power is your wish, I'll grant it but your money will vanish.
In many Unix shells, non-root users will have a \$ prompt. But if you "su" to root, the \$ disappears and your prompt becomes #.
Substituting for that which can be negative,
# can substitute for "no.", which is an abbreviation for number. But "no." can also be negative (the opposite of "yes.")
I'll always remain completely positive.
One could say that # is made up completely of +. Four plus signs all connected to each other.
I can be very important,
In some programming languages, # indicates a comment. Comments are important to developers for documentation purposes.
While at the same time very insignificant.
Comments are insignificant because they are ignored by the compiler/interpreter.
When I contain many things,
The tic-tac-toe board can contain up to nine Xs and Os.
I simultaneously contain nothing.
Tic-tac-toe is also known as "Noughts and crosses". Nought (O) means nothing.
I can signal the beginning,
# indicates the beginning of a comment in some programming languages.
But I can also signal the ending.
Many phone systems will ask you to enter information (such as a PIN), "followed by the pound or hash sign". You press # to indicate the end of that information.