If someone asked this question then there is no one answer for this. SharePoint is a very big system and has plenty of features. So, if we start talking about them then it will take weeks to cover all the features.

But in simple words "Microsoft SharePoint is an enterprise collaboration and content management platform which enables users to connect each other and share the information across an organization. Indirectly which solves the problems in an organization to share the documents, security, business process, maintaining data etc. SharePoint 2010 has rich UI and plenty of features."

SharePoint core features:

1. Sites: You can create a web site in seconds from the UI. You can create how many sites you want according to your requirements and set the different permissions for each site. SharePoint 2010 comes with a rich UI and many improvements to the HTLM as well. The team has replaced all the HTML Tables (in SharePoint 2007) with HTML Div tags (in SharePoint 2010). This makes a huge difference in rendering HTML on your browser, both in performance and time.

And the most important thing is the Ribbon (http://sharepoint-2010-world.blogspot.com/2011/02/ribbon-in-sharepoint-2010.html). The Ribbon is a big revolution, and all the options are on the same page (multiple menus are combined into the Ribbon). The Ribbon properties change dynamically/automatically, depending on what context you are in. There is no need to leave the current page to do something. Everything is on the page, and you can do the stuff that's needed. With this rich UI and ribbon support it is very easy to create and manage content in the site.

SharePoint 2010 sites can be browsable on any browser. The only limitation is it will not support IE6 (I believe no one is using IE6 still). So, we can call it Multi-Platform. This will be very good for developers and users who have their own impression and habits regarding browser usage. You might prefer IE9 and Mozilla. Earlier versions of SharePoint don't work as well with browsers other than IE. So, that problem is resolved in this version.

SharePoint 2010 supports Cloud computing as well. So, you can host it in the general environment or in the cloud computing environment to give access to internet users. This is again a major feature included in SharePoint 2010.

2. Communities: It now includes more Social connections. You can connect to an employee [colleague] or users through user profiles, my sites (where each user has their own customizable SharePoint site), and people search. This way you are connected with others and can share information very easily. Another thing in the communities section is "Informal Knowledge". This includes tagging, rating, and feedback features. For example, if many users are searching for a common document, then we could tag that document with relative keywords. Now it will come up in the top search results. This way users get whatever they want very easily. Another option is rating. You can give a rating to a particular document or item. 

3. Content: As we discussed in earlier sections now with SharePoint 2010, creating or managing content is very easy with the new rich UI and ribbon. This is very easy for admins to find the content in the site and to make changes. You may feel that this is same as editing MS Office files like Word and PowerPoint in your browser. SharePoint 2010 manages huge and complex Metadata across site collections very easily.

4. Search: Search is a very big improvement in SharePoint 2010, compared to previous versions. The way the search has improved is unbelievable and gives the exact search results for what the user is looking for. There are many ways how the team improved search. Here are the main ways:

• Improved relevancy: When a user is typing the search word, SharePoint detects what the user is trying to search for and gives the exact result that they are looking for.     
• Refinement & Related Searches: For each search word given it searches for the related keywords as well and gives them in the result set. If you search for "John", then it gives search results which has "John" on top and then "Jon", "Don" etc. after it. Because Jon matches with 3 letters Don matches 2 letters, etc.
• Wildcard search: This is an already known feature and does not really need any special explanation.

5. Insights: This mainly covers the business process in an organization. The perfect implementation to this is workflows. SharePoint designer workflows are improved and many activities have been added to the 2007 designer activities. With these we can solve many problems without writing custom code. And by writing custom code, you can create your own activities easily. Along with these there are improvements to the Excel Services and charts. In this version of SharePoint, the team integrated Visio services. You can create a workflow in Visio and then import them to SharePoint directly. It is one of the fantastic features in this new version. The main advantage is someone who is good in Visio also has the option to do some stuff that will be useful in SharePoint.

6. Composites: This concept comes from the Mash-Ups: Pulling information from different data sources and combining it single place. So, we can achieve this in 4 different ways:

• SharePoint Designer 2010
• InfoPath 2010
• Business Connectivity Services [BCS]
• Access Services

So, finally, we can say SharePoint includes these abilities:

1. Quickly create and manage content in your organization.
2. Collaborate in new and easy ways through social capabilities.
3. Use search to easily find relevant content.
4. Streamline the business process in your organization.
5. Visualize the information or big data.

This article should help you understand the basics of what SharePoint 2010 is. Before we end this concept, I want to mention that SharePoint 2010 only supports 64-bit. (There is no 32-bit anymore.) So, all the infrastructure and the software (Windows Server 2008, Sql Server, etc.) should be 64-bit.

Source: http://sharepoint-2010-world.blogspot.com/2011/02/what-is-sharepoint-2010.html