KARLA is a catalogue portal that lists all holdings in the University Library (books, journals, online resources and more) as well as articles and other items from the EBSCO Discovery Service, which are mainly available in full text. Some of them however are not available at the University Library and will have to be ordered via interlibrary loan. You should always start your search with KARLA before moving on to other resources such as databases.
To get a broader picture of the academic literature on a certain topic (including sources that are not available in Kassel), you will have to use academic databases. The University Library of Kassel gives access to more than 6,000 databases. A lot of them are already included in KARLA, but for specific search approaches, it is better to use the database directly. There are a number of important databases that you should consult when searching for academic literature for term papers, theses etc. Often these databases provide full text links to scientific papers. Please note that most databases are only accessible within the University Network or via login when working from home. There is a list of all databases available at Kassel University in the Database Information System (sorted by discipline, in German).