What Is the State to the Radical Left?
The radical left typically considers the "state" to be a governing body that one
class uses to force citizens of the state (the members of other classes) to
behave in certain ways. So for radical leftists, a truly
democratic society, where all people are included (or able to choose
to participate) in decision making and where all people have equal
footing when dealing with these decisions, would not have a state, even if there are laws and a military (most consider these necessary evils, but, fingers crossed, a revolutionary society might gradually make these irrelevant).
Why Is the State Important to the Radical Left?
This question about the state and what to do with it after seizing power defines the split between anarchists and communists.
Anarchists fight for a communist society,
but they believe that the state should be dissolved (in other words
fully democratized) immediately after the revolution succeeds in toppling the previous regime (or very soon after). Anarchists will very often point to repression in
really existing socialist societies (like the USSR or Revolutionary Cuba) as evidence that the state is dangerous to keep around, even in the hands of leftists.
Many communists will point to the few instances where revolutionary anarchism has taken place (the Paris Commune, the Spanish Revolution, parts of the Russian Revolution) and talk about their inability to defend themselves against attack, something anarchists often agree with and have worked on solving.
Communists, on the other hand, usually fall somewhere near the
idea of having a "dictatorship of the proletariat" to transition from a capitalist state, through a socialist state, and into a communist lack of state. While
that term sounds scary (with the word "dictatorship" conjuring visions of the worst assumptions about communism), it merely means that the government should be
under the control of working people, a democracy for and by them, to
complete tasks that lay can the groundwork for communism. A dictatorship of the proletariat
is considered socialist, because there is still a state
and probably some leftover class divisions, but the economy is placed under
democratic control of the workers.
In striving for socialism, however, we are convinced that it will
develop into communism and, therefore, that the need for violence
against people in general, for the subordination of one man to another,
and of one section of the population to another, will vanish altogether
since people will become accustomed to observing the elementary
conditions of social life without violence and without subordination.
-- Vladimir Lenin, The State and Revolution
Compromise on "the State" for Unity or Not?
There are other, less general philosophies on the radical left (left communism, council communism, anarcho-syndicalism, and others) which meet somewhere in the middle between the enormous labels of communist and anarchist, and they typically have positions on the state which seems a little communist for an anarchist and a little anarchist for a communist.
That being said, some radical leftists consider the differences between anarchists and communists to be too great, making unity on the radical left impossible. Indeed, some revolutions and conflicts have seen anarchists and communists fighting each other (a fun fact to enliven any Left Unity group meeting).
Still others are optimistic and see a lot of good to be learned from both sides and believe that any mass movement will require the best of both to succeed (only to be called a Stalinist by anarchist comrades and a liberal by communist comrades).