SharePoint 2007

Microsoft SharePoint is a popular web platform developed by  Microsoft  for small to large organisations.

What is SharePoint?

​Microsoft SharePoint 2010 makes it easier for people to work together. Using SharePoint 2010, your people can set up Web sites to share information with others, manage documents from start to finish, and publish reports to help everyone make better decisions. Learn more about SharePoint Capabilities

Why Use SharePoint?

The capabilities of SharePoint 2010 work together to help your company quickly respond to changing business needs. Using SharePoint 2010, your people can share ideas and expertise, create custom solutions for specific needs, and find the right business information to make better decisions. For IT, SharePoint 2010 helps you cut training and maintenance costs, save time and effort, and focus on higher business priorities.​

Really nice tutorial and link is  http://mysharepointwork.blogspot.in/2010/05/sharepoint-2007-development-tutorial.html?showComment=1347639674597#c3083807319272441197

Lets Start with what do you need for your development machine

1. First and foremost, you need Windows Server 2003 R2 with Windows SharePoint Services 3.0 Installed. Then you need to verify that you have Microsoft SQL Server 2005\2008 Developer Edition installed, before you began installing Microsoft Office SharePoint Server 2007 also Known as "MOSS".

Other quick way to start with development is to download SharePoint 2007 VHD from Here and run it with a software called Microsoft Virtual PC. This VHD Contains Microsoft Office SharePoint Server 2007 and Microsoft Office SharePoint Server 2007, you will however need to install VS 2005\2008 to get started.

Once you are all set with the machine you should consider installing Visual Studio Extensions for Windows SharePoint Services 3.0 from Here. This will provide you with various SharePoint Templates to get started.

Let's discuss the knowledge Part - First you need to be familiar with 12 Hive and the Solution structure of SharePoint.

12 hive - SharePoint 2007 have a 12 hive structure which basically contains various files and folders that make SharePoint Interface. The files\components that you will develop will also be deployed under this folder structure.

Solution Package and Feature - Every component that you will develop for SharePoint should be in the form of a Solution Package. These packages are nothing but a .cab files which contain other files that needed to be deployed on the SharePoint server. These Solutions packages represented in SharePoint as .wsp files, also allows you to package Features and defines important metadata about those Features. Besides, Solution Package "Features" is also an important concept in SharePoint development.
Feature is a functional component that can be activated and deactivated at various scopes throughout a SharePoint instances. Almost, every link, button or functional components in SharePoint is a Feature. Take it as a component that can be turned on\off when required. To understand the relation between 12 hive and the .wsp see
12 Hive Folder Structure

Now, lets discuss about SharePoint 2007 Object Model - In SharePoint 2007 Object Model there are two important namespaces .

1. Microsoft.SharePoint namespace - This is the root namespace for all WSS objects. A developer would always use Micorsoft.SharePoint.dll namespace for creating WSS compatible components like webparts,features and solution packages.

2. Microsoft.Office.Server namespace - This is the root namespace of all Office Server 2007 (i.e. MOSS) objects. A developer will use the Microsoft.Office.Server namespace while accessing SharePoint 2007 server (or MOSS) specific components like BDC,Enterprise search etc.

Creating WebParts - Lets Start with creating a very basic webpart using VseWss. This webpart will display some data from a SharePoint list using GridView control on a SharePoint Site Page. The webpart is created as a feature, that means you can activate\deactivate this webpart in your SharePoint site and is deployed in the SharePoint environment as a solution package.
See detail Step-by-Step example Create and Deploy Webpart as Solution Package