Onwards means to continue (often meaning to continue going forwards). An example is "from this time onwards I'm busy." There's the saying "onwards and upwards" which means "continue being successful or making progress". / Forwards and backwards are usually much more directional, as they specify a direction. As a simple example, say I'm telling someone how to park a car, "you just need to drive forwards, then go backwards in to that park." You wouldn't say "You just need to drive onwards,.." Another example is "I look backwards/forwards." / You don't have to use these adverbs. You could say "She ran". Or to add more detail, you could say "She ran ahead", "She ran back to the building." If you're going to specify a direction, you'd use forwards backwards, or a synonym (similar word) to forwards or backwards. / I hope this answers your question, if not tell me :).