[chalk5]

I know that the term selectivity can be applied to a number of different reactions. 

Please explain to me what it means. 
Why are some reactions more selective than the other?
What are some examples?
What's an example of a selective reaction?
What's an example of a reaction that is not selective?

Thank you all in advance!

[PhDoc]

You're certainly correct about the multiple applications.

Please look up the following terms in your textbook:
1. regioselective
2. regiospecific
3. stereoselctive
4. stereospecific

Now, some words about selectivity in terms of general reactivity. Let's use the example of free radical halogenation of tert-butane. If we use chlorine as our source of halogen radical, what we get as our major product, presuming mono-halogenation, is 1-chloro-2-methylpropane (1º halide). The chlorine radical abstracts a methyl hydrogen, and the resultant radical is chlorinated. Now, if we use bromine as our source of halogen radical, what we get as the major product, all things being equal, is 2-bromo-2-methylpropane (3º) halide. Bromine selectively abstracted the 3º hydrogen over the 2º hydrogen.

Why is this so? Chlorine radical is more reactive than bromine radical (kinetics vs. thermodynamics), hence it is less selective. Let's look at this in terms of an equation where "≈" means directly proportional:

                           1
reactivity  ≈  -----------
                    selectivity

As reactivity of the radical increases, the selectivity decreases (chlorine radical). As the selectivity of the radical increases, reactivity decreases (bromine radical). You'll find this equation holds true in many areas of reactivity vs. selectivity.

Get to know what this means in terms of free radical halogenation first, then apply this equation to the following scenarios:

1. Reaction of Grignard reagent with acid chloride vs. Gilman reagent (use your text);
2. Reaction of alkynes with molecular hydrogen under catalysis of Pd/C vs. Lindlar catalyst;
3. Reaction of N(i-Pr)2- with 2-bromobutane (E2 affording 1-butene) vs. ethoxide/ethanol/heat with 2-bromobutane (E2 affording 2-butene).

In every example you'll note the selectivity comes from "reagent control" or choice of catalyst. If any of these reactions are unfamiliar to you, please visit your professor during office hours.

The topic you've posted is enormous... I mean HUGE. It's impossible to address every scenario because selectivity may be introduced to nearly any type of reaction. It will always factor down to one of three things:
I. reagent control
II. substrate control
III. choice of catalyst or temperature or solvent

Other terms such as (a) kinetic vs. thermodynamic control; (b) steric effects; [c] electronic effects; and (d) stereoelectronic effects may factor into I-III.

I hope this helps.