Methinks there's many that will disgaree with the above assertion: look at Dawkins' work on the selfish gene, or the extended phenotype, I'm sure they might point out. Much more famous.
But for me the notion of a meme (pronounced so that the word sounds like "jean", or "gene" in point of fact, if you weren't aware) is the best of the lot: an idea gene, an inheritable, transmissible semantic virus, able to pass from carrier to carrier, to infect person after person.
Many aspects of our lives can be explained by our cognitive bodies (our minds or our "phenomenology") being infected with virulent memes of one kind or another. Dawkins used the example of religion. In memetic terms, religion is a widely successful meme because, despite many good rational reasons to the contrary, people still profess to believe in a supernatural deity such as God or Allah. Daniel Dennett once - in his great, funny way - caricatured human minds as Joycean machines or "meme dungheaps", which is a mischievous image I love.
Just like their biological counterparts, and as Dawkins first explained, memes are inherently selfish. They're not interested in humans or their minds. They're only really interested in their own replication, i.e. in making more copies of themselves. Just as genes are careless of the phenotype (or body), so memes are careless of the phenomenology (or mind).
What sorts of thing are memes?
What are they made of, theses memes, if they're real? Answer, they're not made of anything. Unless you want to start me talking about consciousness and the physical basis of it.
So, religion is a meme, and a particuarly virulent and effective replicator too. What else? Sex is not a meme: more to do with genes, the biological cousins of memes. But pornography is a meme. Hunger is not a meme, but democracy probably is. Freedom is a meme, and so is Coca-Cola.