Different kinds of ship propulsion. CO is Combined, G is Gas turbine, A is And, so COGAG (Combined Gas and Gas) is a setup with one lot of gas turbines for low speed and a big gearbox that lets a second set of gas turbines join in for full power. The L is there to say whether the drivetrain is 'lectric or mechanical. CODLAG is Combined Diesel-Electric and Gas - you have diesels through an electric drivetrain for efficiency and a GT that gets fired up for more power when you want it. The T23 frigates have this - 4 diesels and 2 GTs. CODLAD is the same idea but with two different diesel engine types rather than diesels and GTs.
If it has an O rather than an A it means the two sets of engines can't run at the same time. The first lot of T22 frigates were like that with R-R Tynes for cruise and Olympus for boost, later they got Speys rather than Tynes and COGAG rather than COGOG.
IEP is Integrated Electric Propulsion; whatever the prime movers, diesel, GT, nukes, they all generate electricity and put it into a miniature onboard grid, and propulsion is by electric motors that draw from the grid like everything else in the ship. T45 destroyers* and the carriers do this.
*notoriously it didn't work very well.