The most common use of the word poop these days is as a synonym for poo. It can be a noun (syn. with feces) or verb (syn. go potty). It is most often used by small children or dog owners (where it has been given a boost by the phrase "Please scoop your poop!") This most likely derives from Webster's second definition below (to break wind).
A poop is also a stupid, boring, or annoying person. Somewhat related to the term party pooper.
And finally, poop is slang for information or news; the straight poop, poop from the group, get the real poop. In this case, poop is a mild replacement for shit. Probably also related to poop sheet.
Also see pooped.
http://dictionary.reference.com/search?q=poop&r=66
http://4mermarine.com/USMC/dictionary.html#pissc
http://www.acronymfinder.com/af-query.asp?Acronym=POOP&String=exact&p=ol (below)
Acronyms: Perl Object-Oriented Persistence; Procedural Object Oriented Programming; Plain Old Orbix Protocol; Perl Object Oriented Programming; Pre-Owned Office Products; Post Object Oriented Programming; Poor Operating on Packet; Pipe Organ Owners and Players; Pooled Out of Process; Pseudo Object Oriented Programming; Propane, Oxygen, Oxygen, Propane (mnemonic for safe order for turning on torch); Process Oriented Observation Program; Persistent Object Oriented Perl; Permanently Out Of Print.