Usually the differences are very minor and Translating from the original language to English can wipe away those differences.
Though sometimes the story can become very different - For example a LN can become more 'PC' so it appeals to a wider audience so violence/sex is toned down. And on rare occasions the story can become very different for no discernible reason at all.. For example: Death March to the Parallel World. The Light Novel (and the Manga/Anime since they are based on the LN) is very different from the Web Novel.
A Web Novel is basically the Author Creating a story and posting it on the internet. A Light Novel usually takes the Web Novel, Gives the Author an editor to help them polish the story and publish it in a paperback version.