A SERM is an acronym for Selective Estrogen Receptor Modulator. SERMs bind with estrogen receptors, blocking some estrogen actions while at the same time permitting others. This means their action is different in various tissues, that's where the term "selective" comes from. They selectively inhibit some while stimulating other estrogen-like actions. Compared to aromatase inhibitors, or AI's, which block the conversion of androgens into estrogen. In other words, AIs prevent estrogen from being produced and SERMs prevent certain receptors from receiving and responding to estrogen.
Letrozole is a SERM, Aromasin is an AI.