There are technologies and skill sets that web developers must know for creating web applications.
Server-Side Scripting Languages: The knowledge of server-side scripting language like PHP, Perl, Ruby, ColdFusion, Java, or ASP separates front-end developers from those who make full web apps.
Object Oriented Programming with Coding Architecture: OOP web application development helps in re-using code and keeping files and code in order. Anyone can learn OOP through tutorials and practice. It is also important to learn about programming architecture for maintaining your code consistently and efficiently.
Know Database Designing: When you are planning to use a database, be sure you know how to structure it well. In India, web development experts never miss that.
Integrate with Third Party Services: Now days, almost every web application development has some sort of integration with Facebook, Twitter, Google, PayPal, Amazon etc. Thus, the developer should how to integrate a web app with these services through API or by simply logging in to a service.
Responsive or Mobile Development: It is important that every web app has a mobile version or tablet version that accommodates many screen sizes.
Basics of Good Business, UI Design, and Strategy: If an app is poorly executed from a design and business standpoint, it will never succeed.
Version Control: There are many open-source version control systems available, such as Git, SVN, and CVS, including many free tools for versioning your web apps.