Both are concessive markers. The subjunction (subordinating conjunction) "obwohl" means "although". It is generally used in a dependent clause. Example: "Obwohl es regnet, spielen die Kinder draußen." (Although it is raining, the children are playing outside.) The adverb "trotzdem" means "anyway, nevertheless". It can be used as a concessive adverb in a main clause. Example: "Es regnet. Trotzdem spielen die Kinder draußen." (It is raining. Nevertheless, the children are playing outside.) You CANNOT say: "Es regnet. Obwohl spielen die Kinder draußen." However, in some cases the word "trotzdem" functions as a subjunction (subordinating conjunction) but you find this only in regional and colloquial language. Example: "Die Kinder spielen draußen, trotzdem es regnet." In standard German, you would say: "Die Kinder spielen draußen, obwohl es regnet." (The children are playing outside, although it is raining.) Remember: 1. obwohl = although (subordinating conjunction) 2. trotzdem = nevertheless (adverb)