Hi Naresh,

Greetings to you!

Apps in Salesforce are a collection of tabs to achieve certain functionality. The difference between the custom app and console is the look and feel (tabs access) and the level of access to the Salesforce platform.

Custom App:
A custom application is a collection of tabs, objects, etc that function together to solve a particular problem. 

Custom applications are easier to build, as console development over and above simple configuration requires Visualforce, Apex and sometimes javascript skills.

You can also build your own on-demand apps by grouping items into new custom apps. A custom app consists of a label, a description, and an ordered list of items, which often includes tabs. You can also add custom logos and branding to your custom apps.

In Salesforce Classic, custom apps are listed in the Lightning Platform app menu, which is a drop-down list that displays at the top of every page.
In Lightning Experience, you can find your available custom apps in the App Launcher.

Example: Apps are built as a unit for complete process/functionality like Sales, Marketing or Service. Meanwhile, apps can be custom or out of the box and representation of tabs for users to access.

More information: https://help.salesforce.com/articleView?id=dev_tabsets.htm&type=5

Console App:
A console application uses a specific Salesforce UI - the console. Console applications are intended to enhance productivity by allowing everything to be done from a single, tabbed, screen.

Salesforce Console apps are a tab-based workspace suited for fast-paced work environments. Manage multiple records on a single screen and reduce time spent clicking and scrolling to quickly find, update, and create records.

The console is perfect for Sales and Service folks. Support agents can use multiple applications at once and keep the context of cases as priorities change. Sales reps have all the tools at their fingertips to cultivate leads, close deals, and manage relationships.

Example: What if a service rep is busy working on some issue and all of a sudden Account/Contact details are needed for customer validation then without losing his/her current work and time, Service rep can access Account/Contact by opening in a new tab.

More information: https://help.salesforce.com/articleView?id=console2_about.htm&type=5

I hope it helps you.

Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in the future. It will help to keep this community clean.

Thanks and Regards,
Khan Anas