The main difference between "regular" daily vitamins and prenatal vitamins is the latter have folic acid (or a greater amount of it) in them, which during can aid implantation, help prevent fetal heart problems, and help lower the risk of preterm birth/birth defects and pre-eclampsia in pregnant women. Having enough folic acid while TTC is mostly important on the chance that you'll get pregnant and not know it for a few weeks, but there's some evidence that it helps people produce follicles and helps ovulation disorders.
To be honest, I'm not -entirely- sure if it's 100% necessary if you eat a diet already high in folic acid (leafy greens, citrus, some beans, folic acid-fortified foods). If not, either prenatals or a folic acid supplement might be worth looking into. Looking into it, it seems getting slightly too much folic acid (not a crazy amount, of course; overdoing anything is usually never a good idea) rarely does any harm while too little can, so I just went on and switched to prenatals while TTC. Before that the doctor actually prescribed me a separate supplement to take with my daily multivitamin.
