In terms of Robonomics Network agent is a program module that uses IPFS or blockchain or both interfaces of the network and does some actual work. Usually it's represented as a ROS package and it may connect (but not necessarily) a real cyber-physical system to the Robonomics Network.
It is a combination of a physical mechanism that is usually called a robot and a program algorithm that controls the behavior of the mechanism.
It is a short form for Decentralized application. Usually it is a single page web based application that helps to interact with an agent.
According to the official documentation "IPFS is a distributed system for storing and accessing files, websites, applications, and data". For more detail how it works go to the official website.
A lighthouse is an autonomous workflow that allows us to distribute the running time of providers that serve a single broadcast channel.
For more information read Robonomics Whitepaper section 5.2.
Ethereum based blockchain network with Proof-of-Authority consensus owned by Airalab.