Sharepoint - What exactly is SharePoint?

SharePoint allows you to share files (it's a document library), but it's also

• a document management system allowing versioning, check-outs, etc
• a CMS which can contain lists, blogs, wikis, etc
• a notification system for advising when items have changed or are added
• an extensible development platform allowing web parts that work much like desktop applications
• a tool for data integration i.e. reports and dashboards

SharePoint is so many things that it is really very hard to categorize but I'd probably call it a 'portal' (a word that is vague enough to cover just about anything), as most places seem to refer to it.

Sharepoint is more than a fileshare. It´s more like a Application Plattform with a set of prebuild features and the Abbility to implement your own logic.

Have a look here for a list of features and compare it to what you can do with dropbox (Btw Skydrive is implented in Sharepoint aka Office 365)

In an (larger) Enterprise settings - SharePoint is viewed as a 'knowledge' management & sharing platform that provides communication, collaboration & "content curation". The latter term is for aggregating content found dispersed across the Enterprise and brought together by search & metadata tools. This post gives a good oversight about it - Sharepoint and Enterprise 2.0: The good, the bad, and the ugly. (it's a bit dated but is still a very relevant reading).

In the above answer - Kirk Broadhurst's point of 'extensible development platform' is very important. SharePoint provides for integration points to external systems like databases, LOB apps like HR & Supply Chain via SharePoint's APIs, BCS, REST et al. Without these, SharePoint and its content will remain a standalone "stove pipe" in the environment.