Consider the reaction :
$\mathrm{RCHO}+{\mathrm{NH}}_2{\mathrm{NH}}_2\to \mathrm{RCH} = \mathrm{N} – {\mathrm{NH}}_2$

What sort of reaction is it ?

Given reaction is an example of a nucleophilic addition-elimination reaction
In the first step, hydrazine is added to the carbonyl group. This is a nucleophilic addition reaction.
RCHO+NH_2​NH₂​→RCH(OH)−NH−NH₂​
In the second step, elimination of water is taken place as shown below:
RCH(OH)−NH−NH₂​→RCH=N−NH₂​+H_2​O