"He is a boy."
Translation:Он - мальчик.
"Есть" is the infinitive form of the verb meaning "to eat" conjugated as follows: Я ем, ты ешь, он/она ест and so on. You can find all the conjugations at: http://masterrussian.com/verbs/est_sest.htm.
At the same time, the word "есть" is also used to mean "there is". I believe (although I could be wrong) that this is because at some point in the past "есть" was also the infinitive form of the verb meaning "to exist". Thus, phrases such as "У меня есть кошка" (I have a (female) cat) translate literally as: "With/by me exist (female) cat".
Hope that helps, and have fun!
Ест is the third singular person present of the verb есть (comer), ex: он ест (he eats).
Есть, beyond meaning "to eat", also has existence sense meaning "there is, there are", and possessing meaning in the construction у + genitive + есть. Ex: У меня есть (I have).
And their phonetic difference is that ест ends with hard т and есть ends with soft т because of the soft sign ь. It's kinda hard to explain it here. May you google it for more explanation or we could see another way to explain it to you.