When you say that something is O(N) or O(N^2) or whatever, you are saying that as N changes then the resource in question (time or memory normally) changes with that relation to it. So if something is O(N) and N doubles, then the time taken doubles.
To be pedantic, that's not true. O(N) means that the growth is at most linear. O(N^2) means that the growth is at most quadratic. This means that any algorithm that is O(N) is also O(N log N) and O(N^2).
If you want to express that an algorithm is linear (and not worst case linear), the correct function is use is called Θ.
Note also that hash lookups are, worst case, Θ(N). There's always a chance that all hash keys map to the same value, resulting in a linear list that needs to be searched.
Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
Read Where should I post X? if you're not absolutely sure you're posting in the right place.
Please read these before you post! —
Posts may use any of the Perl Monks Approved HTML tags:
 a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
Outside of code tags, you may need to use entities for some characters:

For: 

Use: 
 &   & 
 <   < 
 >   > 
 [   [ 
 ]   ] 
Link using PerlMonks shortcuts! What shortcuts can I use for linking?
See Writeup Formatting Tips and other pages linked from there for more info.

