Excellent answer already from grfrazee but I wanted to add a few details.
Although there is some overlap in sizes with this kit (and many others) often the bits don't overlap in function, so the size overlap is quite deliberate.
They only duplicate function if you think of drilling one 1/2" hole as the same as drilling every 1/2" hole. For example, if you want a shallow, flat-bottomed 1/2" depression in a piece of wood the Forstner is the only choice here. If you want to drill 1/2" though-holes in a beefy workbench for bench dogs or holdfasts then the auger would be the one to pick. For just drilling a simple hole through the thickness of a board then the spade bit is sufficient (although both the Forstner and the auger bits can be used for this as well, where appropriate).
Note that the bits in this set are not truly Forstner bits. As indicated by the toothed edge these are multi-spur bits of a similar type but not the original Forstner design.
As mentioned Forstners are perfect for flat bottomed holes (that is except for the central depression left by the "gimlet point"). They also drill equally well irrespective of grain orientation. In addition, they are suitable for boring partial holes, i.e. where you want to drill a scallop from the edges of a board.
Important to stress that these are basically intended for use only in a drill press. You can use them with a handheld power drill, but it's risky and not recommended practice.
The central threaded point on these is called the lead screw. In addition to allowing the bit to be centred accurately on a mark when commencing drilling these actually pull the bit through the wood, reducing the force necessary for drilling quite considerably; and as a result great care should be exercised if drill speed is high.
Due to their overall form they excel at drilling deep holes with dead-straight sides, the entire length of the bit if needed.
Note: not well suited to boring into end grain as the lead screw can easily get clogged with swarf and cease to draw the bit into the wood.
These get a bit of a bad rap for leaving surface tearing around the hole and this can be a problem, but not all spade bits are created equal.
On cheap ones the flutes to either side are dead flat across their tops and these are more prone to splintering surface wood fibres. Better ones have angled faces on the flutes (e.g. 10°) and may additionally angle upwards from the centre slightly to further improve cutting performance. Other well-designed spade bit have little spur-like projections at the outside shoulders to help cut a sharp-edged hole; where a purchased bit doesn't have these they can be filled into the profile using a needle file or slipstone.
One advantage of spade bits over the previous two types is that the angle of the hole can be changed slightly during drilling if needed, something the design of an auger bit will specifically not allow.
Brad-point or lip-and-spur bit
As with the spade bits there are some variations in design for these. Some have a central point only, with straight or angled shoulders, others have a spur on each outside edge.
No real overlap with other bit types here as these are the bit of choice for drilling deep, smaller-diameter holes in wood.