Dental in phonetics means that the tip of the tongue is against the back of the upper teeth, or even further forward. The T D N and L of French, Spanish, and Italian are dental, but in English and German these sounds are alveolar, made further back on the alveolar ridge.

The difference is slight, except that a few languages contrast them: in Dravidian and Australian Aboriginal languages and several Sudanese languages like Dinka and Nuer there are both dental and alveolar versions of these sounds, and they can change the meaning of a word. For example, Bidyara of Queensland gundu 'away' versus gundhu 'go across', where D and DH represent the contrasting stops. In such languages the dental stops may be interdental, that is with the tongue actually between the teeth. An interdental sound may be heard in English in a strong Jewish accent: it is more noticeably different than the lamino-dental (upper teeth) variant of French or Spanish.

The initial consonants in English 'thin' and 'this', both written TH, are dental. The T sound may be dental by assimilation in a word like 'eighth' which is pronounced with T followed by TH.

Dental affricates (the T+TH sound but as a single fused sound) are very rare, but I have a vague recollection some Iroquoian language, possibly Mohawk has it.