Filmic - new addition, replaces base curve. Fine grained control of midtones, shadows, highlights. Brings saturation back into highlights and shadows
Crop and rotate - the most valuable tool! Get rid of any distractions around the edge of the frame & make sure the horizon is straight.
Tone curve - lab unlinked channels - global saturation and contrast control. Also change color of objects if necessary
Color Zones - awesome for tweaking specific colors
Equalizer - often use clarity preset. I shoot a lot of gritty, paint peeling type of scenes, and some local contrast makes the image pop wonderfully. Tweak until content, then back off the settings a bit. Easy to go overboard.
Retouch - get rid of little distractions in the frame.
Vignette - all my photos have the corners darkened slightly. I use the lightness mode and the blend at 12%
Spot removal - probably removing in favor of retouch… But it removes spots.
Exposure - bump the over all lightness of the image a bit. Useful with base curve and filmic
Lowpass - usually to brighten shadows a la Harry Durgin
Highpass - Sharpening a la Harry Durgin
Please note that if you’re new, there is no need to learn every module right away. Master a few modules and then add new ones as you need! Also make sure you get a good grasp of the masking, parametric and drawn, as this is what really sets darktable apart from other editors IMO