Here's a quick explanation for some of the parameters:
Here you select the shader, depending on the use of your material. Most of the time this would be either LightmappedGeneric for brush textures and decals, WorldVertexTransition for displacement blend materials or VertexLitGeneric for model materials.
This is the toolbar that contains shortcuts for the most used parameter groups, I use only these in 95% of cases. Other groups can be found under Add... menu item, or you can add any missing parameters under the Additional parameters tab.
Top-to-bottom these groups are: Transparency, Detail, Color, Other, Reflection, Self Illumination, Specular and Misc.
Opacity controls the overall opacity of material, transparent and alpha test use diffuse alpha channel. Transparency is smooth but has sorting issues that alpha test solves, but has hard edges - used for foliage and such. You can control the cutoff point with threshold. Additive doesn't need an alpha channel, black is see through and white is solid (like Photoshop). Check Decal if you plan to use the material as decal or overlay.
Should be pretty self explanatory, adds a detail texture over the whole material, scales higher than 1 mean smaller than diffuse and lower than 1 mean larger than diffuse. Note that non-uniform scale is not implemented yet. Default detail blend mode is similar to overlay, meaning neutral gray has no effect.
Tints the whole material. Color is for LightmappedGeneric and color 2 for VertexLitGeneric. Blend tint by base alpha uses diffuse alpha channel as color tint mask (works only on models in CS:GO).
Here you can add a lightwarp texture, control reflectivity of the material or use seamless scale for displacements. Seamless scale doesn't work with normal maps, only ssbump.
This if for cubemap reflections, you can use auto generated cubemaps with the Use cubemap checkbox or specify your own custom cubemap in the Reflection: line. For the reflection mask you can also specify separate texture in the mask line (only works with no bump map) or use either diffuse (base) or normal map alpha channel.
Tint controls the intensity of the reflection with the slider or its color with the pick color dialog. For saturation 1 is default and 0 is black and white, for contrast 0 is default and 1 is most contrast. Fresnel controls the visibility of reflection depending on the angle; 1 means default behavior, 0 is no reflection when looking perpendicular and full when looking horizontal.
Light influence and Anisotropy are new parameters introduced in Nuke update, explained here: http://blog.counter-strike.net/workshop/maps.php#anisotropic
For glowing textures, uses diffuse alpha channel by default. You can control the power with the tint slider.
Phong reflections on materials. This one is different depending on selected shader. This is for LightmappedGeneric, only works in CS:GO. http://blog.counter-strike.net/workshop/maps.php#phong With amount you can control the color as well as power of the specular reflection. Since it doesn't take the color of the light environment into account, it looks best if you use the color of the sun. Last value should generally be left alone as it brightens and darkens the whole material. Glossiness controls the size of the highlight, smaller values mean bigger highlight.
Brush specular also uses diffuse luminosity for mask as default, you can control the contrast and brightness with the next set of sliders. You can switch the mask to base alpha or normal alpha mask (uses same parameter as reflection, you can only use one).
VertexLitGeneric specular group:
Fresnel controls the visibility of the highlight at certain angles. X is looking 90° at the material Y means 45° and Z is 0°. Better explained here https://developer.valvesoftware.com/wiki/Phong_materials#Phong_Fresnel_ranges I find a good value to start with is 0.5 0.8 1. Tint works the same as in Reflection group, you can also use diffuse color to tint the whole phong highlight. Unlike brush specular, it takes into account the light color, so it's best to leave it white/grey unless you want a special effect like colored metal.
Glossiness controls the size of the highlight the same way as in brush specular with the exception that you can provide your own glossiness texture. You can make the highlight brighter with the boost slider.
By default, specular uses the normal map alpha channel as mask, but you can also use diffuse (base) alpha or its luminosity as masks.
Contains tool texture and keywords lines, as well as other parameters that you may find useful in certain circumstances. Model checkbox only needs to be use on materials other than VertexLitGeneric. Vertex alpha and Vertex color make the surface derive its alpha and color values from per-vertex data provided by the engine. No decals means no impact, blood or other such decals can show up on the surface of this material, useful for chain link fences. Ignore Z ignores the engine object sorting and displays material in front of everything. No fullbright and No lod are used in UI textures (such as L4D posters). No fog means the material is not affected by the environment fog.
There's no set values for any of these parameters so it's best to test yourself and see how it looks in game. Use the mat_reloadallmaterials command to quickly see changes after you save.
Hopefully that covers the basics, if you have any questions feel free to post them here.