In the last episode of season four 'Witch Way Now?' the Angel of Destiny (Protectors of the Grand-Design so it isn't altered) revealed that Piper was pregnant.
Wyatt was prophecized as a girl, and to be the most powerful being of all time. His name was given as 'The Twice Blessed'. The prophecy foretold the twice blessed would be born on the day Magic died and would restore all magic as soon as he was born. Wyatt was born and magic was safely restored.
It is revealed that in an alternative future, Wyatt is the most powerful being and rules the entire world. He is also evil and caused the great 'Event' which wiped out most of humanity and all of the magical community. His younger brother, Chris, went back in time to change this. He successfully did so, as Leo killed Gideon, who was responsible for Wyatt's change.
Wyatt starts using his powers with his own will in this series, at first on his younger brother Chris, then for himself. This causes problems for Piper as she tries to discipline him not to orb things all over the place in case someone sees. Leo has a different method of teaching Wyatt and soon Piper accepts that he is gifted and coming in to his own. We also see Wyatt from the future, who is a healthy young man with incredible powers. Wyatt also begins to talk during this season.
Wyatt is little boy now and is not focused on that much throughout the season. When Leo had to be taken away by The Angel Of Destiny, Wyatt blamed himself. But after his Aunt Phoebe talked to him, he realised it wasn't his fault. Wyatt's powers were also stolen by the Hollow-possessed Billie and Christy, who used his powers to battle The Charmed Ones. As he was trapped in the manor with no way of orbing out, his little brother Chris did it for him.
Wyatt will be the most powerful being to ever walk the earth, and be the oldest of Piper and Leo's three children. The half-elder and half-witch, Christopher Perry Halliwell, and the half-witch and half-mortal, Melinda Prudence Halliwell. He is also taught by Paige on how to accomplish his whitelighter duties.
Like other magical witches from his family line, Wyatt has the power to write and cast spells, make potions and scry for certain types of magical activity, in addition to his other innate magical powers.
Forcefield Generation - first manifesting itself while still in his mother's belly, Wyatt has the power to generate a powerful blue field around himself and others, suitable for protection from physical harm and magical attack, and can also be used offensively like a battering ram, shown in the series to be able to send even groups of demons flying back with seemingly little effort.
Telekinetic Orbing or "calling" - in addition to his more traditional telekineses, and a product of his witch/whitelighter crossbreeding, Wyatt has the power to telekinetically orb things to him by saying the name of (or "calling" for) them to come to him, much like his aunt Paige. Wyatt's power to do so, however, is seemingly much stronger than hers, having shown that he has the power to summon Excalibur to his hands from the manor's attic while being far away.
Advanced Molecular Combustion - much like his mother, Wyatt seemingly also has the ability to excite or speed up molecular bonds, causing solid matter to fly apart in a combustive reaction. This power was only seen once in the series and it was never fully explained whether this was an advanced state of his mother's power or more like the increased telekinetic waves Prue was able to generate in the future.
Wyatt was also shown using several other magical abilities that were never fully explained, such as the power to increase and effect the volume and tone of his voice, as well as several magic powers he was shown using in the alternate future where he was evil, such as the power to create and throw energy balls.
Weapons and Equipment
Aside from being seemingly well-versed in the use of potions and magical assundries like daggers and athames, Wyatt also has access to the mystic sword Excalibur. Passed along to his mother while he was in utero, Wyatt is said to be the destined recipient of the powerful blade by the Lady of the Lake. Excalibur makes its choosen bearer (Wyatt and Piper being the only two currently) invincible, being shown to have the power to block and even redirect magical attacks and powers. The sword will return to its bearer's hand at will and cannot be summoned, pulled or called away from its weilder if they do no wish it.