What is the difference between tank atomisers and cartomisers, or are they the same deal?
Atomizer is the heating element in ecigs/mods, basically consisting of a coil, wick, and head unit to hold the assembly and connect to the device. However, the terminology of the implementation of an atomizer is what gets confusing.
Here is a rundown of your different types of non-RDA/non-RBAs:
What you might see referred to as general "atomizers" on a vendor's page are normally an atomizer + small tube with no fiber filling, for dripping - otherwise called dripping atomizers (keep in mind these are not RDAs, or rebuildable dripping atomizers, which are sometimes called just dripping atomizers also). These generally have a very small capacity, like 3-5 drops of liquid.
A "cartomizer" is an atomizer with a tube, called the cartridge, attached; cartomizer = cartridge + atomizer. Cartomizers sometimes have shorter cartridges, and may look similar to the above mentioned dripping atty's. However, all true cartomizers have polyfiber filling which the coil runs through and which absorbs and holds the juice. A "cartotank" refers to a cartomizer+tank system, in which the cartomizer inserts into a tank, and is resupplied with juice held by the tank. If you get frustrated with leaking problems common to other types of atomizers, this is a good way to go.
A "clearomizer" is essentially an atomizer with some kind of clear tank system. Usually when the term "clearomizer" is used, this is referring to the lower end units with plastic tanks and the like; ex. CE4s and CE5s are referred to as clearos, as are things like iClear and iClearB. Larger capacity clearomizers are sometimes referred to as "clearotanks". Models like your Protank and Nautilus are technically clearomizers, but are also referred to as clearotanks or more recently, "glassomizers", which specify their pyrex or glass tanks. Everything here can be called a "clearo" and be correct.
Also realize sometimes people lazily just use all the terms interchangeably even though they know what they mean, and pretty much anything can be referred to as an "atty", though this is typically said when referring to RDAs/RBAs.