
A filling is to restore the cavity caused by caries. When you have a cavity it is a bacterial infection in the tooth and it decays the tooth and makes a hole within the tooth. Prior to putting a filling, all caries must be removed. Such that it prevents the cavity from getting bigger. Fillings are one way of restoring a damaged tooth. However, there is a limitation to the filling procedure. If the cavity is too big or has not much retentive tooth structure remaining after removing all caries, doing a filling procedure may not be indicated and it may need a metal crown. Cavities get bigger over time if you don’t take of it, you may end up getting more extensive and expensive dental work such as root canal or extraction.
Remember, it is better to take care of caries when it is small because it does have better prognosis after the treatment in terms of sensitivity, etc.
There are several types of fillings. Mainly they are divided into direct or indirect fillings. Direct fillings are the ones that you are commonly aware of. It is directly put on the chair-side using either resin or amalgam fillings. Indirect fillings are inlays and onlays where an impression is taken and sent to the lab for the fabrication and cemented on the second visit.
Filling Types
Theoretically speaking, in-direct fillings are supposed to be ideal but it comes with a very high price comparable. Depending on the dental office you go, you may be paying somewhere in between $700 and $1100 for each inlay and onlays. Maybe because of that, it is very uncommon to see them in practice. Not only that, there are a few steps where human errors can cause indirect filling failure. During the indirect filling process, error in impression and dental lab error may cause inaccurate margins on the indirect fillings, and it may not be as good as it proclaims to be.
• Indirect Fillings – Caries are excited and prepped in such a way indirect filling fabricated by a dental lab can be cemented and delivered. Common materials are Gold or Porcelain. Porcelain can look better but can wear down the opposing occluding tooth.
• Composite Resin Fillings – These are what most people referred to as “white filling”. They are cured (set) by a curing light so you don’t have to wait for chewing after the procedure. It is tooth colored filling so it is more esthetical, however, it may cause a bit more sensitivity to compare to amalgam filling when the cavity is big.
• Amalgam Fillings – These are what most people referred to as “silver filling”. It is not very esthetical, so commonly used on back molars. Although it is not very cosmetical, it may be a better option when the cavity is big close to the nerve, as it can cause less sensitivity post operation. The amalgam fillings are not fully set within 48 hours so you may not chew on something very hard within that period, otherwise, the filling can break.

Not all caries show up on x-rays. Most of interproximal caries (caries between teeth) show up on x-rays but if caries is located on the front or back side, they don’t show up until it gets really big. Also typically, carries on the top surface of tooth does not show up on x-ray until they get significantly big, in which case, the intra-oral exam is done by your dentist to detect any abnormalities on each tooth.
Remember, it is better to take of caries when they are small, otherwise, it may cause discomfort even after getting the filling procedure. Also, you are potentially risking yourself of losing the tooth.

Reference:
https://en.wikipedia.org/wiki/Dental_restoration
https://www.mouthhealthy.org/en/az-topics/d/dental-filling-options
https://www.mouthhealthy.org/en/az-topics/c/composite-fillings
https://www.ada.org/en/member-center/oral-health-topics/amalgam