A floater is the most versatile line- you can use it with dry flies on the surface, and also reach moderate depths with your leader/tippet to fish streamers, nymphs and wets sub surface. How deep you get with a floating line is a function of the weight of the fly (or adding split shot to the leader or tippet), length and diameter of the leader, and also the current speed of the water since the faster the water the more tendency for the leader to ride up as "drag" is introduced. To fish nymphs deep with a floating line, many people use an "indicator", which is a thing that is attached to your leader that floats (like a bobber), and the weighted nymph is hung on a length of tippet below that. A rule of thumb for moving water is to set the indicator above the fly 1.5x the depth of the water. This will let the nymph get close to the bottom. You watch the indicator for hits, and to control drag by mending the fly line to ensure a good drift.
A sink tip will let you get a little deeper. Sink tips are fly lines with a sinking portion at the front end, and the rest of the line floats. It can vary from 10-25 feet, and you can get them in different densities ("sink rates" for any given fly line weight). Some have tips that sink very fast, some that sink slower. How long a sink tip, and what sink rate depends on how deep you want to get. Longer sink tips with fast sink for deep water, short sink tip and slow sink for shallower water. The floating portion of the sink tip allows you to "mend" the floating portion of the line in moving water to control drag.
Sinktips are useful for fishing moving water especially in pools and deeper water stretches where fish might be holding deep, and fishing from shore in lakes and ponds. They're excellent to use in early and late season with unweighted streamers.
In addition to floating and sink tip lines there are full sink lines that sink throughout their entire length. These also come in a variety of sink rates but are generally better for lakes fishing from a boat or float tube. All things being equal, a full sink will get deeper than a sink tip, but they're harder to manage in moving water.
In addition to learning the mechanics of casting, one of the keys to catching fish in moving water whether you're fishing dries, wets, nymphs or streamers is learning to control "drag" by "mending" Here's a good article explaining the hows and whys: Fly Fishing, Fly Presentation, Mending - MidCurrent
Hope this helps.