Nucleophilic substitution

$CH_3CHO + HCN$: $CN^-$ adds to the carbonyl carbon; this is nucleophilic addition.
Propene + water in acid: $H^+$ attacks the double bond first; this is electrophilic addition.
$RCHO + R'MgX$: the carbanion-like R′ adds to the carbonyl carbon; this is nucleophilic addition.
Alkyl bromide + $NH_3$: the nucleophile $NH_3$ attacks the carbon bearing bromine and displaces $Br^-$ as the leaving group; one group is replaced by another, so this is nucleophilic substitution ($S_N2$).
Hence the nucleophilic substitution reaction is the reaction of the alkyl bromide with $NH_3$.

In aryl halides (I and II) the C–X bond has partial double bond character due to resonance and the carbon is $sp^2$ hybridised, so they are much less reactive towards nucleophiles than alkyl halides.
Among the aryl halides, the two electron-withdrawing $-NO_2$ groups at the ortho and para positions in (II) stabilise the intermediate carbanion, so (II) is more reactive than the unsubstituted halobenzene (I): I < II.
The alkyl halides (III and IV) react readily; taking the reaction through the carbocation ($S_N1$) pathway, the tertiary halide (III) forms the more stable tertiary carbocation and is more reactive than the secondary halide (IV): IV < III.
Combining these: I < II < IV < III

In a nucleophilic substitution, a nucleophile attacks the carbon bearing the halogen and replaces the halide ion.
RX + KOH → ROH + KX: the nucleophile $OH^-$ replaces $X^-$ on carbon. This is nucleophilic substitution.
RX + Mg → RMgX is the formation of a Grignard reagent (insertion of the metal into the C–X bond).
2RX + 2Na → R–R + 2NaX is the Wurtz reaction (a coupling reaction involving the metal).
RX + $H_2$ → RH + HX is a reduction.
Hence the nucleophilic substitution reaction is RX + KOH → ROH + KX.