OK, I’m still learning this, but they say you only really know it when you can explain it to somebody else. So I’m practicing on you @Audiofreak 
Lux measures Lumens. Lumens are just all visible light. Lux meters are used for people lighting, but gives a general idea of how bright a light is for plants, just based on Lumen intensity. This can work if you already know the light is for plants.
Plants don’t used the whole visible spectrum however, and the spectrum of light important to plants (400-700nm) is called Photosynthetic Active Radiation (PAR). So technically there’s no such thing as a “par meter”, since PAR isn’t a metric, it’s a type.
Photosynthetic Photon Flux Density (PPFD, or PPF) is the metric used to measure PAR. More specifically, how many photons within a given spectrum hit an area each second. PPFD is changed with a light dimmer, or adjusting the distance to the plants.
The Daily Lighting Integral (DLI), which this thread is about, is a number standard for a 24hr period of PPFD. The DLI will change based on lighting schedule. A 12/12 schedule will have a lower DLI than an 18/6 at the same light intensity (PPFD), because the sum of light for the day will be less. This makes DLI the culmination of measurements and is the most complete representation of how much light plants are receiving.
None of that tells you which meter to get, but it might help in shopping for one…