There are two approaches to building a user facing web application: Salesforce Visualforce together with Apex (think HTML and Java) or Lightning. Lightning is a Javascript framework. You are also able to import and export data into or out of the platform via the REST and SOAP APIs and also create integrations with other software in their App store like Docusign. An example of this is to have a Salesforce Development Services create a front-end in lightning that invokes an approval process in Salesforce and then makes a call on Docusign to generate a Docusign envelope with approvers and signers.