There is no point you can compare sharepoint with asp.net.Both are different .The only relation is sharepoint is devoloped using .net framework.asp.net is used to customize the features of sharepoint.
Comming to what is sharepoint, it cannot be defined in a single line, it has a lot of features if you start talking about it.
Simply to tell about sharepoint, It is a colloboration of enterprises and a platform which manages their content ,whiah allows users of different level in the organization to communicate each other resulting an effective bussiness process.
It also enables best security ploicies and systematic maintenance of data among the organisation.
For better information with eloberated details go through this link:
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