Which substance/ion is oxidised and which substance/ion is reduced in the following reaction?
$H_{2}S+2FeC{l}_3\to 2FeC{l}_2+2HCl+S$.

$H_{2}S+2FeC{l}_3\to 2FeC{l}_2+2HCl+S$.

Oxidised: $H_{2}S$ The oxidation number of S increases from -2 to 0. Thus, S itself is oxidised and acts as reducing agent.

Reduced: $FeC{l}_3$ The oxidation number of Fe decreases from +3 to +2. Thus, Fe itself is reduced and acts as oxidising agent.

Note:

Any species that is oxidised acts as reducing agent.
Any species that is reduced acts as oxidising agent.