GO to tech net and read through the entire SharePoint section, these are SP basics, i would really suggest creating few sample project/sites on SP first.

user controls  are like u r creating ur own functionality though groupiing up the asp.net controls.
Web part are the one tht are personizable.
Site definition  consists of xml ,aspx pages and the navigation structure.
Site template is based on site  definition .

If u want more info plase go through moss sdk

please tell me is there any books to read basics of sharepoint

i think inside windows sharepoint services 3.0 by Tedd Pattison will the best book to start.
Moss sdk will be the next

Qn2 and Qn3 are very obvious.

Qn1: Diff bet'n Usercontrol and Webpart.

UserControl:
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1. Can not be used on any general Sharepoint page. You gotta code a custom page layout  and embed the usercontrols.
2. Do not have the ability to communicate/connect to other user  controls on the same page.

Webpart:
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1. Can use on any page through out.
2. Webparts can communicate/connect to other webparts and make use of the master/detail.

sharepointsolutions.com/.../...raining-videos.aspx

nice difference  mentioned between user  controls and web  parts. However, i think user controls  can be also added to web part pages. We just need to write one generic web part loader which can render the User control .

So even if we have user control, we could display it anywhere like web part.