Definition:Subset

A set $$S$$ is a subset of a set $$T$$ if and only if all of the elements of $$S$$ are also elements of $$T$$, and it is written $$S \subseteq T$$.

This can also be read as "$$S$$ is contained in $$T$$", or "$$T$$ contains $$S$$".

If the elements of $$S$$ are not all also elements of $$T$$, then $$S$$ is not a subset of $$T$$:

$$S \not \subseteq T$$ means $$\lnot \left( {S \subseteq T}\right)$$

For example, if $$S = \left\{ {1, 2, 3} \right\}$$ and $$T = \left\{ {1, 2, 3, 4} \right\}$$, then $$S \subseteq T$$.

So, if we can prove that if an element is in $$S$$ then it is also in $$T$$, then we have proved that $$S$$ is a subset of $$T$$.

That is, $$S \subseteq T \iff \left({x \in S \implies x \in T}\right)$$.

Superset
If $$S$$ is a subset of $$T$$, then that means $$T$$ is a superset of $$S$$, which can be expressed by the notation $$T \supseteq S$$. This can be interpreted as "$$T$$ contains $$S$$".

Thus $$S \subseteq T$$ and $$T \supseteq S$$ mean the same thing.

Also see

 * Proper subset