what is the difference between lo sé and yo sé?

Literally, lo sé means I know it, whereas yo sé means I know. But practically, I think they can be used interchangeably. Perhaps lo sé could be used when you want to emphasize what you know...I know IT, whereas yo sé could be used when you want to emphasize that you know whatever is known...I know.

But again, I think they are interchangeable.

So basically I can say 'yo siento' (for Im sorry) as well as, 'lo siento' and it wouldn't really matter?

No, I don't think you can. Lo siento means, "(for it) I am sorry." I think that is the common expression.

Saying "yo siento" probably does not mean what you think it does.  You have to say "lo siento" to effectively say I'm sorry.  The verb "sentir" means "to feel" so saying "lo siento" literally means "I feel it" and that somehow means "I'm sorry".  It would be strange to simply say "I feel" since the verb seems to demand a subject.

Definitely: "Lo siento" is the formula way to I'm sorry. An interesting question is when you would use "sentir" as opposed to "sentirse". I would venture the following:
- siento que voy a poder...: 
- me siento [adjective]

Corrections please if I'm wrong!

Hola a todos,

I came across something regarding sentirse vs. sentir. Sentirse: "How" you feel, Me siento bien. "What" you feel, Siento el frío. "How" reflexive. "What" not reflexive Kinda handy!

Saludos,

Ricardo

¡Gracias Ricardo! Eso es lógico para mi. 

"How" you feel (sad, happy) comes from within = reflexive.
"What" you feel (cold, hot, etc.)  comes from without = not reflexive.

Thanks Ricardo!

De nada amigos.

Thanks everyone!