Instant conditioners tend to contain: Silicone, various oils that do not penetrate hair, and conditioning salts (such as behentrimonium chloride, stearylkonium chloride, centrimonium chloride) that improve manageability and softness by attaching to the surface of the hair cuticle.
Deep conditioners tend to in addition contain: Hydrolyzed keratin, collagen, soy, oat, wheat, silk proteins to name a few; as well as amino acids, silk peptides, conditioning oils that penetrate the hair ( coconut, olive, avocado etc), and often manufacturers will include heavier silicones like dimethicone. Indie companies tend to add things like avocado butter, shea butter etc, for a thicker product.
The former is to add manageability to the hair after a shampoo session. Some people also enjoy using these as co-wash cleansers especially when they are silicone free. They can also be used as light detanglers.
The latter is to increase strength and elasticity as well as add manageability. People tend to leave these in for at least 30 minutes with heat for best results. The idea is to condition from the inside out in a manner of speaking. The deep conditioners tend to contain ingredients that penetrate the cuticle. This is called adsorbing and it is more efficiently done when heat is applied using body heat, a hooded dryer, a steamer, a thermal cap etc.
HTH!