Immediately under the Basic section is the Tone Curve tab. Essentially the same as a Photoshop curve, modifications to the curve will brighten or darken areas of the image. The default RGB which affects all channels can be changed to affect individual RGB channels which then affect color as well. Options are the default Linear (straight line) or Medium Contrast or Strong Contrast which simply introduce S shaped curves to enhance image contrast.
The displayed dialog shows the alternative Region curve which is accessed by clicking on the curve icon at the left. This dialog breaks the curve into four regions with sliders. This is easier to deal with in learning how curves work as the changes are dynamic and each area being changed illuminates as you move the sliders. However, this is RGB and cannot be managed in individual color channels as the Point Curve dialog allows. The Region section is therefore aimed at adjusting overall image contrast rather than color as it does work on the luminosity.
Note the three triangles at the quarter points at the bottom of the curve display. They can be shifted in order to modify the regions affected by the four divisions. In either curve option any custom curve can be saved if you wish to apply it easily to another image or save it as a preset modification.
I encourage Lightroom users to use the Tone Curve section to accustom themselves to the curves concept as the technique is very valuable in the event you move images to Photoshop. The curves adjustments in Photoshop are among the most powerful controls you have over color and density in an image. Many of the other dialogs introduced over time in Photoshop and Lightroom are similar in their effect on the image but are presented to the user in different forms. Even hue and saturation can be modified with Curves. Add blend modes to the tool and you have a very powerful means of controlling your images. Changing the blend mode to luminosity, for example, allows you to change the brightness and/or contrast of the image without shifting the color.
The HSL (Hue, Saturation and Luminance) and Color tab show two different ways to represent the same modification options. The dialog at the right is the HSL option and shows the sliders for all the different color ranges with the option to change the Luminance values of each. Clicking on the Saturation or Hue tabs gives you control over those alternatives. The Color dialog alternative allows to you to choose a specific color range instead and then offers the HSL sliders for that color. Choosing the "all" color wheel in the color dialog opens all of the color options at once on the screen with three sliders in each. Which dialog you might prefer to work in is a personal choice and may depend on what you intend to manipulate in any particular image. The sliders all do the same thing and are simply presented to you in different forms.
Split Toning is the application of a color hue to a region of the image. The Split Toning tab in Lightroom allows you to do this in a simplistic fashion in that it applies the toning in two regions, highlights and shadows, simply dividing them with a balance slider. The balance slider allows you to decide where the division is between highlights and shadows. There is no option for a neutral middle ground which you might do with selections or gradient maps in Photoshop. However, there is still a lot of creative toning that can be done to both color and B&W images.
Split toning is often done to B&W images at low saturation levels. At saturation levels much over 25% the effect can be a bit overdone and obvious. The image at the right has been toned with a warm hue in the highlights and a cooler hue in the shadows. In this image the effect is stronger than I would normally use in order the make the effect obvious. Good split toning is much like vignetting in that it should be felt more than seen. Roll over the image to see the original monochrome version before toning. When choosing your hue it is helpful to have the saturation high so the color is obvious and then you can reduce the saturation until the effect feels appropriate.
Split Toning can help you impart personalization to your images. Toning can also be added to either highlights or shadows alone and the balance slider used to manage the point at which the tones are added. This singular effect may be more useful in color than B&W. In color the application can be overdone easily and a subtle approach is recommended.