When we have a German word composed of two or more terms, is there always an 's' between the two words? Someone who lived in Germany for a long time told me that there is always an 's', but as far as I remember there are words that don't need 's' in between. However, as my level is not that good in German, I can't think of any examples.
The s is called "Fugen-s" and is not always used. There is no general rule for its use, so its appliance is a question of feeling.
It is generally used when the first word ends with -tum, -ling, -ion, -tät, -heit, -keit, -schaft, -sicht, -ung as well as with Verbs used as Noun ending with -en:
Altertumsforschung, Frühlingserwachen, Kommunionsfest, Realitätsverlust, Einheitsfeier, Heiterkeitsanfall, Eigenschaftswort, Ansichtskarte, Erinnerungsvermögen
Essensreste, Lebensfreude, Leidensweg, Redensart, Schlafenszeit, Sehenswürdigkeit, Sterbenswörtchen, Wissenslücke
Schadensersatz, but: Schadenfreude
(and a lot more, not necessarily following the rule above)
There is no Fugen-s in
Weltkugel, Nachtzug, Fruchtsaft, Kammerdiener, Lageplan, Redezeit, Musikzimmer, Naturschutz, Schurwolle, Räuberhauptmann, Ritterburg, Steuererklärung, Zigeunerjunge, Nebelhorn, Paddelboot, Pendeluhr, Wendeltreppe, Nebenstraße, Ladenpassage, Rasenfläche, Wagenachse, Grußkarte, Lastwagen, Sitzkissen, Putzmittel, Herzkammer (and many more)
Note that the use of Fugen-s's differs in different German speaking Countries.
See Wikipedia: Fugenlaut (in German) for more information