Significance of Reversible and Irreversible Process

I would like to ask the following questions:

1. What is the significance of reversible and irreversible nature of thermodynamic process? (I understand that reversible processes are quasi-static, happening infinitesimally, but what is the direct outcome of being reversible and irreversible?)
2. What does the "reversible" work and heat flow (Wrev and Qrev) signify? It is hard to understand why there are things like "lost work" due to irreversibility of a process.

Searching in the textbook and online learning sites do not seem to help - it would be great if it can be explained in simpler terms and with examples/analogies.

Thanks!

• What do you mean by "signify" here? – ACuriousMind Apr 2 '15 at 12:23
• By "signify", I mean the "results of the gas being rev./irrev., in practical terms". For example, why the need to specify "reversible" work/heat flow in certain situations? (e.g. dS = dQrev/T instead of just dS = dQ/T) – CHECK009 Apr 2 '15 at 13:39

Reversible processes are important because they are related to the efficiency of a process. Take for examples a pair hotplates, one at 100C and one at 0C. In a theoretically ideal setting you could extract some work $W_0$ from this system until the two hotplates reached equilibrium. Then we would say the process is reversible, because in the same ideal world you could input $W_0$ into the system and once again create the same original temperature gradient in the hotplates.
In the real world, because of friction, poor design, and other factors, you might only extract $.7W_0$. We would say that some work in the system is lost to irreversible entropy. This is entropy which can't be reversed unless you add in extra work from the outside. You can partially reverse this system, but because you weren't able to extract all the possible work, you won't be able to reach the original temperature gradient. You might only get one hotplate to 80C and the other to 20C.