The Hidden Shirk + What is Shirk?

The Prophet (sall'Allaahu 'alayhi wa sallam) said: "Shall I not inform you of what I fear for you more than the maseeh-id-dajjaal? It is the hidden shirk. It is when a man stands up for prayer, and he beautifies his prayer because he sees the people looking at him." (Sunan Ibn Majah Vol 2, 3389)

The Prophet (sall'Allaahu 'alayhi wa sallam) taught the specific du'a for riyaa:
"Allahumma innee a'oodhu bika an ushrika bika wa ana a'lamu, wa astagh-firuka limaa laa a'lam."
"O Allah, I seek refuge in You from committing shirk with you knowingly, and I seek your forgiveness for that of which I am unaware (the shirk that we may commit unknowingly)."