Give an NL-algorithm for complement of 2-SAT
Question1: What is the difference between 2SAT and the complement of 2SAT?


The set of all strings that do not describe satisfiable 2CNF formulas.


  Question2: It is known that NL is contained in P, but what we know about P over NL? can be said that an algorithm that runs in P, uses NL space?


We don't know anything significant beyond that $\mathrm{NL}\subseteq\mathrm{P}$.

It doesn't really make sense to ask if "an algorithm... uses NL space". You can ask if an algorithm uses logarithmic space and if it's nondeterministic, but there's no such thing as "nondeterministic space" per se. By that, I mean that the storage space itself isn't nondeterministic: nondeterminism is a property of the algorithm that uses the space.

It's certainly not true that an algorithm that runs in polynomial time necessarily uses logarithmic space: it could use polynomial space. It's possible that every deterministic algorithm that runs in polynomial time could be rewritten as a nondeterministic algorithm that uses only logarithmic space but we don't know if that's true or not. If it were true, we would have $\mathrm{P}=\mathrm{NL}$.

Note that, formally speaking, $\mathrm{NL}$ and&nbsp;$\mathrm{P}$ are classes of problems, not algorithms.