RJ is an acronym that stands for registered jack which is the name that the cable connectors start with. The two most common jacks are the RJ 45 and RJ11 with each having specific purpose. The main difference between the two lies in where they are used. RJ45 jacks are used with internet cables in networking whereby different computers and other network components to each other whereas the RJ11 is used in connecting telephone units. The following is a picture of rj45 waterproof connector.
The other noticeable difference is that the RJ45 connectors accommodate eight wires hence are bigger than the RJ11 connectors which can accommodate four wires. The following is a picture of a RJ11:
An ATA port on the other hand is a device that is used for connecting traditional analog telephone and other related devices to digital telephone systems. The ATA provides dial tone, DC power, caller ID data and other standard telephone line signaling to a telephone that is connected to a modular jack. An ATA is connected between an IP network and an existing telephone jack to provide service that is indistinguishable from public switched telephone network providers on all the other telephone jacks in the residence. The following is a picture of an ATA port: