TITLE: What is the intuition behind short exact sequences of groups; in particular, what is the intuition behind group extensions?
QUESTION [22 upvotes]: What is the intuition behind short exact sequences of groups; in particular, what is the intuition behind group extensions?

I'm sorry that the definitions below are a bit haphazard but they're how I learnt about them, chronologically.
In Johnson's "Presentation$\color{red}{s}$ of Groups," page 100, there is the following . . .

Definition 1: A diagram in a category $\mathfrak{C}$, which consists of objects $\{A_n\mid n\in\Bbb Z\}$ and morphisms $$\partial_n: A_n\to A_{n+1}, n\in \Bbb Z,\tag{6}$$ is called a sequence in $\mathfrak{C}$. Such a sequence is called exact if $$\operatorname{Im}\partial_n=\ker \partial_{n+1},\,\text{ for all }n\in \Bbb Z$$ [. . .] A short exact sequence in the category $\mathfrak{C}_{\Bbb R}$ of right $\Bbb R$-modules is an exact sequence of the form $(6)$ with all but three consecutive terms equal to zero. [. . .]


Also, ibid., page 101, is this:

It is fairly obvious that a sequence
$$0\longrightarrow A\stackrel{\theta}{\longrightarrow}B\stackrel{\phi}{\longrightarrow}C\longrightarrow 0$$
is a short exact sequence if and only if the following conditions hold:
$\theta$ is one-to-one,
$\phi$ is onto,
$\theta\phi=0$,
$\ker \phi\le\operatorname{Im}\theta$.

I'm reading Baumslag's "Topics in Combinatorial Group Theory". Section III.2 on semidirect products starts with

Let $$1\longrightarrow A\stackrel{\alpha}{\longrightarrow}E\stackrel{\beta}{\longrightarrow}Q\longrightarrow 1$$ be a short exact sequence of groups. We term $E$ an extension of $A$ by $Q$.


Thoughts:
I'm aware that semidirect products can be seen as short exact sequences but this is not something I understand yet. My view of semidirect products is as if they are defined by a particular presentation and my go-to examples are the dihedral groups.
Please help :)

REPLY [25 votes]: Let's say you want to classify all the finite groups (up to isomorphism). You know that there are simple groups, the groups which have no non-trivial normal subgroups. You can think of these as the "atoms." Now you want to classify all the finite groups, not just the simple ones. You might hope that an arbitrary finite group is a product of the simple ones, but unfortunately it's not that... simple. 
For example, the dihedral group of order 6 is "built out of" $\mathbb{Z}/2\mathbb{Z}$ and $\mathbb{Z}/3\mathbb{Z}$ in the sense that it has a normal subgroup isomorphic to $\mathbb{Z}/3\mathbb{Z}$ and the quotient is isomorphic to $\mathbb{Z}/2\mathbb{Z}$, but it's definitely not their product. So to finish the classification problem, you need some set of rules for forming "molecules" from the atoms.
The "molecule" rule is equivalent to this: given two groups $G$ and $Q$, classify all groups $E$ such $G$ is a normal subgroup of $E$ with the quotient isomorphic to $Q$. This is exactly the extension problem. If we have an effective way to calculate this for every pair of groups, and we also know all the simple groups, then we have solved the classificaiton problem.