Geez, Lizzie, just because she/he? is a Yankees fan...
Valjean is an escaped convict who is chased throughout the play by Javert (it's more complex than that, but that's the gist). Valjean gets put into a position where he feels he's wronged Fantine causing her to die (it's more elaborate than that, of course) and so he adopts Cosette. The Thernardiers are inkeepers who are less than moral who Fantine had sent Cosette to live with while she worked to pay for her. Cosette grows up, and falls in love with a student revolutionary, Marius. Eponine is a girl at the barricade where Marius is fighting who is in love with him as well, though her love is not reciprocrated. Gavroche, as Lizzie said, in the book is the Thernadier's son, but it isn't mentioned in the play. He is a boy who is also fighting at the barricade. The play has a lot of big themes, though I would say the most important is what the word "justice" means (but that's subject to debate). It's also about "being yourself" (sorry, random Simpsons joke).
Hope that helps.
Hi, Shirley Temple Pudding.