The main difference is that light novels are in prose form, while manga typically use pictures.
Additionally, light novels tend to be focused on a coherent narrative present tense with the exception of some first-person narratives and flashbacks. This means that light novel readers don’t need to worry about underlying issues like continuity errors in relation to ages or time periods when they’re reading from start to finish, which comic readers need not do so for their own enjoyment.(i.e.: if one volume is set 10 years after another where both main characters are 16 years old then how old will they be now?) Manga may focus on narrative (though most often it depends on the order of publication)