I'll take them in reverse.
An imaging device, usually a film or platesetter, exposes film or plate to light. Let's call the smallest addressable area of exposure a "speck" (because the words 'dot' get thrown around too much, and confuse people). So our platesetter or filmsetter can reliably make small specks of exposed area on plate or film.
3) usually in FM screening, we are using a very small dot, of a consistent size. The number of these small dots within an area produces the same effect in our eye as making the dots larger and smaller, as in AM screening (Amplitude Modulation). In FM screening, we're changing the Frequency that these tiny dots are scattered across film or plate. What's the actual size of these dots? They're tiny, and measured in Microns (short for micrometer, μMeter, 1/1,000,000 of a Meter). The common sizes are 10μM or 20μM.
2. DPI. Those 'specks' I described, used for making an image on plate or film, are the smallest addressable parts of an image on a plate or film. By addressable, I mean that the laser light can be simply turned on or off at these locations. How many of these specks can be found across a certain length? Thats DPI, Dots Per Inch. By using the word 'dots' though, it makes for confusion. Think 'specks' at this point. Common DPIs used in printing range from 1200 DPI to 3000 DPI. I'd have to say 85% of everything I see is 2400 DPI. Creo heads, for example, are almost all set to 2400 DPI. Other recording devices use zoom lenses to allow for different DPIs.
Another term for DPI is 'Resolution', as in "how finely can we resolve this image?". Think of a Photoshop image, and using the Zoom In function over and over until you see big pixels on the screen. You have reached the limit of your 'resolution' and can resolve it no better.
3. LPI. Lines Per Inch. In AM screening printing, we are printing images represented by dots of varying size. The dots can be different shapes like circles, ellipses, or squares. Each dot in the formation has an area of space, like a grid, that can be completely empty, completely filled, or something in between. Let's call that grid space a 'cell'. How many of these cells are there in an Inch? That's LPI, Lines Per Inch. Different needs call for different LPI from fine to course. 200 LPI might be for printing a Ferrari brochure on heavy paper. 65 LPI might be for a newspaper or screen printing, where you can actually see the dots with the naked eye.
150 LPI is very common in a lot of Commercial Printing.
If we exposed a plate with a 50% tint in a 1 inch square at 150 LPI square dot screen, with the angle of this screen at 0Ëš or 90Ëš and looked at the results through a microscope, we could actually see and count 150 lines of half filled square dots (in both directions, up and down).
To tie all this together, each one of those lines of square dots is made by a machine that builds those dots with 'specks' of image according to the resolution set, say 2400 DPI (or specks per inch) (see why I don't like to say 'Dot'?)
I know, way too long winded! That's what you get when I'm having my morning coffee.
