First of all, let’s define Business Intelligence. It’s simply trying to make sense of the data a business already has.

As an example, consider a company which sells toys, and stores a record for each toy it sells. This record contains the information of the country in which the toy is sold.

Now, the manager of the company wants to see in which country the sales is higher, so that the next year, the distribution plan would be more efficient for that country. He/She needs a report of the sales figures in different countries. This is an example of business intelligence.

Now to get to this report, somebody has to get the data out of the database (storage place, anywhere, even an Excel file). But wait, what if the total records of the data you have in your company exceeds, say for example, 50 million records? Do you want to query over them each time you want to create that report? Even worst than that, what if your database is under a huge amount of transaction and many records are getting inserted into it, while you try to execute a very costly query upon it?

These problems resulted in some science to grew out of the solutions developers proposed. For example, you might create another database, and run a job each night to replicate these databases, so that tomorrow you can execute your query on a database which is not under live transactions.

Some concepts come to mind here, like OLAP (Online Analytical Processing) vs. OLTP (Online Transactional Processing), Data Warehousing, Data Mining, Cubes, Tools for BI like SQL Server Reporting Services and SQL Server Analysis Services, and many other concepts, which are not related to being a web developer at all.

A BI developer may actually not have any web development know-how at all.

Core skills of a BI developer would be in-depth knowledge of databases, data mining, data warehouses, etc. There’s probably a bunch of proprietary systems (e.g. Oracle BI Suite) that a BI developer would be expected to know (and have specialized in one or two of them).

Domain knowledge of the business in question would be a big plus as well.

A microsoft BI-developer would know most of these:

1. sql server
2. SSIS
3. SSRS
4. cubes and so forth.

SQl server 2012 is the latest and the greatest database from Microsoft.

http://www.microsoft.com/en-us/sqlserver/editions/2012-editions/business-intelligence.aspx

As experts shared their opinion here what BI is.I want to digress here a bit by explaining what are the basic elements and skills they should posses to work in this area.Though I’m not expert but tried to share few ideas what I come across and past few years.Please make your counter idea and let me correct if there is difference in opinion.

A BI developer/consultant should posses these:
Must be aware of few BI tools and be expertise in one of them that he/she can claim his/her hands on experience on that.
Preferably Tableau Desktop is better to start for beginner as it’s easy to learn and demanding these days.Focus on getting knowledge on tool interface then all functions and charts provided.

Later on SAS,Roambi(for ipads),Microstartegy,OBIEE,Cognos,pentaho,Tibco,Qlikview -choose any of them later on.
Apart of that learn a database(prerequisite here is DBMS and Basic SQL queries),prefer to learn DB which is best suited to play with large amount of data,learn any of Oracle,Postgre SQL,SAP HANA,MS SQL Server and last but not least Mongo DB.

One who wants to expand his career in ETL,Informatica is best tool.
Guys who is interested in analytics should be aware of m/c learning,AI,statistics,complex SQL queries,ability to write stored procs etc,as a tool SPSS is best to start with.Later learn “R”(language dedicated for statistics).

Apart of all these get basic awareness of DWH(dataware housing) like cubes,star schema,fact,dimension,OLAP vs OLTP concepts,Objectivse of a BI,ERP,CRM,DWH.

Also learn how to better use MS-Excel because clients usually keep thier data on this spreadsheet and we need to manipulate/model them to make it adeqaute to use.
Integrating/Importing/Exporting data and connecting Excel,DB(MS SQL server),Tableau.Using theobald software for SAP connection to tableau.

And most importantly better communication and interpreting skills to make other understand the things in lucid way.Business domain knowledge helps while developing project which helps you to observe things closely as you are there to make CEO reports,KPI dashboards,quick visuals and better insights.
Above all try to understand data,what they are trying to say that you can transform them in better insights using tools.After all,this is your objective as a BI developer.Along with these keep brush up with SQL area,because its equally important for all related sectors what I mentioned above.

According to me,these are the lethal combination a BI consultant/developer should be.