MongoDB Replication sets are basically multiple mongod processes running on multiple servers that maintain the same data set highly available.
MongoDB is a cross-platform program, commonly referred to as a document-oriented database. It uses a JSON-like documents format.
Apache Tomcat is an open-source technology, which licensed under Apache License version 2. Basically to understand what the use of Tomcat it's a simple HTTP server used for Java Servlet, JavaServer Pages, Java Expression Language.