Mark's answer is certainly true, but there is more to the story.
To find out, we realy need to read this passage in context.
22 Then they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see. 23 All the people were astonished and said, “Could this be the Son of David?”
24 But when the Pharisees heard this, they said, “It is only by Beelzebul, the prince of demons, that this fellow drives out demons.”
25 Jesus knew their thoughts and said to them, “Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand. 26 If Satan drives out Satan, he is divided against himself. How then can his kingdom stand? 27 And if I drive out demons by Beelzebul, by whom do your people drive them out? So then, they will be your judges. 28 But if it is by the Spirit of God that I drive out demons, then the kingdom of God has come upon you.
29 “Or again, how can anyone enter a strong man’s house and carry off his possessions unless he first ties up the strong man? Then he can plunder his house.
30 “Whoever is not with me is against me, and whoever does not gather with me scatters. 31 And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. 32 Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
33 “Make a tree good and its fruit will be good, or make a tree bad and its fruit will be bad, for a tree is recognized by its fruit. 34 You brood of vipers, how can you who are evil say anything good? For the mouth speaks what the heart is full of. 35 A good man brings good things out of the good stored up in him, and an evil man brings evil things out of the evil stored up in him. 36 But I tell you that everyone will have to give account on the day of judgment for every empty word they have spoken. 37 For by your words you will be acquitted, and by your words you will be condemned.”
So if we look at the context of this, we can clearly see what provoked Jesus to say this. It was when the Pharisees were calling the work of the Spirit the work of beelzebul. When Jesus performed the miracle, they denied it as being from God. Jesus explained that 1. it was really stupid to say that the devil would cure somebody since he's the one that cursed them. and 2. The Holy Spirit is the one who was giving Him the power to drive out the deamons, and calling the work of the Holy Spirit evil, is the worst thing you could ever do and you will be held accountable for it. He was making it clear that they had crossed the line from just discrediting Him(they thought he was only a man, so it was nothing for them to speak against Him), to blaspheming the Spirit of God.
So in context, we see that blasphemy against the Holy Spirit is when the Holy Spirit is doing something miracules and you turn around and say that it is of the devil.