Esta(s) is feminine, este/estos is masculine, and esto is unknown/neutral I believe.

It can be a little confusing, I know. First of all, esto/eso/aquello can only be used as pronouns, that is, without a noun. When you say, "what's this?" --> "¿qué es esto?" Este/esta/eso/esa/aquel/aquella can be used without a noun when in English you would add "one"... for instance: "- Which is the book you want? - This one" --> "- ¿Cuál es el libro que quieres? - Este". (the tricky part is that you have to use the -e version when one replaces a masculine object, the -a version when feminine) When you use them with a noun, then the same rule of masculine/feminine applies, and you use este/ese/aquel or esta/esa/aquella. Hope that helps.