FNAC is a procedure which can be diagnostic or therapeutic (in the case of cysts) when patient have thyroid nodule. Basically, this is done to identify what is in the nodule to aid in management and treatment later on. It is a very short procedure (like in 15 mins?) and its usually not painful (only minor pain/discomfort). The pathologist will insert a small needle into the nodule and by using a syringe, to extract the content. This step may be repeated to obtain enough and satisfactory samples. Sometimes ultrasound might be used to aid in the procedure. It is not exactly a "surgery" but more like a procedure I would say (like blood taking, etc...).

Pros:
1. Relatively safe procedure
2. Diagnostic and can be therapeutic
3. It does not take long

Possible side effects include bleeding, infection, uncommonly, damage to the surrounding structure like blood vessels and nerve. I believe your doctor/pathologist will brief you about this in more details.