In Salesforce.org, an app serves as a container. There are tabs, a logo, and a name on them. It's a collection of tabs that provide the Salesforce developer with the features he or she is seeking for. If you need to develop an app in Salesforce, simply follow the steps below:
Setup ->Build ->Create->App-> Click on new
Salesforce has two types of Apps which are:
Custom App
Console App
Also, check out the blog on Salesforce Trailhead.