It will basically be like this:
-Your server will be Forge
-Sponge will be a Forge coremod (aka go into the mods-folder)
-Sponge-Plugins will be… well, plugins (aka go into the plugins folder)
-You will be able to add other Forge mods into the mods folder just like you’re used to with Forge
-As long as none of the Sponge plugins and none of the Forge mods require client-side features, people will be able to join the server with a vanilla client