You can see that aluminum (in the reactant) has an oxidation state of zero. Remember that when an element is 'alone' in a reaction, its oxidation state is always zero. In the case of HCl, H has the oxidation number of +1 and Cl -1, the algebraic sum between these two elements is zero.
In the products, the oxidation state of chlorine (Cl) remains unchanged whereas aluminum (Al) has the oxidation state of +3. Doing the algebraic sum of these ox. states of AlCl3, we obtain: -1 (3) + 3 = -3 +3 = 0. And hydrogen (H2) has an oxidation state of zero.
You can realize that aluminum is oxidating because it's changing its ox. state from 0 to +3 and hydrogen is reducing from +1 to 0, meaning that this is an oxidation-reduction reaction.
There are such types of chemical reactions. For this redox reaction, we can identify easily that the type of reaction is a single replacement reaction. A single-replacement reaction is a reaction in which one element replaces a similar element in a compound. We're replacin