Syrup is a Ponychan meme started by user Ikariuga (who now posts under other usernames and tripcodes) on the 7th of May, 2011.
Syrup became a meme during a possible parasprite thread where the topic was whether or not the ponies in the series menstruated. Because of the ultra-sweeteness of the series, Ikariuga proposed that the ponies menstruated syrup, and the term immediately caught on. Reaction to posting of syrup generally ranges from laughter to mock horror.
It is not commonplace to see it on ponychan since it's died down, but users exposed to and know what Syrup is, often recognise it instantly, and remember it well. However those unaware of the truth about syrup will state that syrup tastes great, as does icing (see below).
Somtimes, syrup is also used instead of the sauce, or the other way around, as they share multiple similarities.
Icing exists as a sub-meme of Syrup. It was coined by Keldeo, formally known as Gary Oak on /oat/, who created a thread asking the question "If ponies menstruate syrup, do they poop icing?". The thread was promptly moved to /ewwww/ and lost forever in the bottomless pit of Ponychan. However, icing still remains the sole substance that ponies poop. Unless you're going by asksweetiepoo's logic, of course.evert