Once the bike is as clean as new it's time to give it a bit of shine [if you wish] and lubricate the drive-train, concentrating on the rear derailleur, chainset, chain and cassette. There is a variety of lubricants depending on your riding needs - dry, wet and all-purpose.
*Dry lube - A light oil, perfect for dry and dusty weather conditions. This lube is less sticky so when used correctly your drive chain will remain pretty clean as it won't attract too much dust and grit. However in wet weather dry lube will soon wash away, offering little protection.
*Wet lube - Thicker textured oil made to withstand wet conditions where mud and grit start to cake onto your bike and its components. Wet lube tends to be stickier, attracting a lot of dirt and dust which can clog up the drive-train
*All purpose lube - Non-specific oil that is an all-round lubricant. It can be used in all weather conditions but won't work as effectively as weather specific lube. You may find it doesn't last very long or offer high-quality protection.
Lubricant eventually wears away and when it does, unprotected bikes parts can start to wear against one another. It is always worth investing for the longevity of your drive-train, saving you money in the long run. It is advisable to have a selection of lubes, they will last you years so it is worth it.
There are spray lubricants that help protect and freshen up your bike whilst adding that just cleaned shine. Many of these have water dispersing action, so they are ideal post bike clean, before lubrication! Muc-off MO94 and GT85 sprays are two popular choices. It is important to note these are not lubricants but will lightly clean and leave a protective film.