BI solutions help companies retreive and analyze key data related to their business processes, and generate reports. What you do is implement and deploy custom solutions for your client's specific needs, and you make use of a BI suite or another (libraries and tools to handle data and dynamically generate reports or other forms of data visualization). There are proprietary BI suites, most notable form Oracle, SAP, Salesforce, and there are also open source suites, like JasperReports, Pentaho, SpagoBI (I reccomend you JasperReports). I don't develop business software myself, but my company offers consultancy and implementations of such solutions.