The alkylating agents are called so because they promote a chemical reaction inside the body's cells, at the very DNA inside the nucleus. These medicines, inside the body, attach an alkyl group to the bases on the DNA strands. This essentially throws a monkeywrench into the core of the cell's ability to reproduce. The chemicals also cross link DNA strands and prevent match-up of analogous nucleotides, which essentially causes a mutation. So just as genetic mutations cause cancer, so can chemicals that cause mutations be used to fight cancer.
In addition to carboplatin and cisplatin, alkylating agents include busulfan, nitrosoureas, cyclophosphamide, and nitrogen mustard. Side effects of all alkylating agents include myelosuppersion (leading to anemia), stomatitis (irritation of the mucous membranes of the mouth), alopecia (hair falling out), lower fertility, and nausea and upset stomach.
In terms of chemical structure, carboplatin is different from cisplatin in that one of the chloride groups is replaced with a cyclobutane dicarboxylate group. When the agent bind to DNA, they form the same reaction products, but the carboplatin reaction is slower than the cisplatin reaction.