What is the difference between a service and an application in Sharepoint 2013?
Service applications are different SharePoint features which you can configure and share among different web applications. There are lot of built-in service applications which you can configure and benefit from.
Applications - if you are referring to custom applications, are the one you can develop using Browser, SharePoint Designer or Visual Studio to introduce new functionality into your solution.
SharePoint facilitates several services such like user profile, secure token, business connectivity and etc.. which are supported at farm level. An application is a web application where it also lies in the same level as services. There you can add site collection, sites and add content. Then the services come and help those applications to function by facilitating with a set of services.
Basically SharePoint Farm has many service applications and can have many web applications as well.
Scaling a SharePoint farm is often achieved by efficiently sharing resources across sites running in different web applications and by offloading processing cycles from front-end web servers to dedicated application servers.
In Old version(2007) we have the SSP concept, But from 2010 & onward we have this new service architecture.
In SharePoint 2010, service applications are used to facilitate sharing resources across sites running in different web applications and different farms. The new service application architecture also provides the means for scaling a SharePoint farm by offloading processing cycles from the front-end web servers over to dedicated application servers.
A key benefit to this new architecture is that you can treat a service application as a pluggable component. After a service application has been installed and created, it can be configured for several different deployment scenarios. In simple farms, an instance of the service application can run on each front-end web server. In more complex farms, a service application can be configured to run on a separate application server or in a farm of application servers.
But for the Applications:
If you are talking about the Web Application then
A Web application is composed of an Internet Information Services (IIS) Web site that acts as a logical unit for the site collections that you create. Before you can create a site collection, you must first create a Web application.
Each Web application is represented by a different IIS Web site with a unique or shared application pool. You can assign each Web application a unique domain name, which helps to prevent cross-site scripting attacks.
But if Application means App development.
SharePoint 2013 is a versatile development platform for building apps and solutions with varying scopes that address a wide range of needs. The SharePoint 2013 developer documentation guides you through the features, technologies, capabilities, and models for development that distinguish SharePoint 2013 as a development platform. Our developer documentation walks you through the essentials of writing your first app, getting started working with the platform, and creating, using and interacting with SharePoint 2013 resources in your own code. We provide in-depth articles about SharePoint concepts, step-by-step how-to task guidance, and code samples to help you begin building apps for SharePoint and SharePoint solutions quickly and easily.