Born in Dulwich England on November 23, 1887, Karloff was given the birth name William Henry Pratt. 

He moved to London to attend London University where he took his first steps towards not a career in acting, but a career in politics by studying to be a diplomat.

For several years he was considered "up and coming" until his big break came in 1931, when Columbia Pictures released The Criminal Code, which catepulted Karloff into the "in-demand" hollywood scene. It was also the year the now legendary "Frankenstein" premiered on the movie screen.
Moving to Canada in 1909, and again to California in 1910, Boris was able to find only small bit parts at the beginning of his film career, of which an aging old man in The Devil ('10) was his first. That was until the '20's, when talking movies came on the scene.  That was when this once ignored actor added his unique voice to his acting talents, giving him better roles and more exposure.
Not only was Karloff now a huge acting success, he was being touted as the next Lon Chaney in the Horror Genre.  In a way, he owes a partial bit of thanks to Bela Lugosi, who was originally pegged to play the monster but instead wanted the lead in a remake to Hunchback of Notre Dame to be called "Quasimodo", which unfortunately was never made. 

 That left James Whale, the director, to search for a replacement, and he stumbled upon Boris at the Univeral Lot and immediately gave him the role that would make him an "overnight sensation".

After roles in several nondescript films, Karloff once again delved into the world of monsters when he played Kharis, in 1932's "The Mummy". At this point, the major hit run in his career would begin as the studio's placed him in most all their now well known horror classics of the 30's. 

The Old Dark House ('32), The Mask of FuManchu ('33), The Ghoul ('33), The Black Cat w/Bela ('34),  and eventually The Bride of Frankenstein ('36).
This gave Karloff a chance to do something he couldnt do in the original "Frankenstein", and that is give more character and sensitivity to the monster.  

Though the film starts with Karloff continuing his murderous pace from the first film, he quickly transforms the creature into a sympathetic, misunderstood man/child.

His last go round in the Universal Classic Horror films was as Dr. Neiman, the mad scientist in "House of Frankenstein".  In this version, he brings the Frankenstein monster (glenn strange) back to life in an effort to rule the world. 

Karloff pretty much saves this movie, which most consider a disjointed effort by Universal.  Nonetheless, Karloffs portrayal of the mad doctor is dead on.

There was a possiblity that Karloff was going to recreate the monster role for Abbott and Costello Meet Frankenstein, but decided against it as he didnt want to deminish what he had already accomplished in terms of the creature.   As for his last turn as an actual monster in the movies, it would be 1956's Abbott and Costello meet Dr.Jekyll and Mr. Hyde, which wasn't exactly appreciated by the critics, but I always found enjoyable.
Boris Karloff continued to act in film and lend his legendary voice to narratives and voice overs, including the Christmas classic How The Grinch Stole Christmas.  His last theatrical effort  was Peter Bogdonovich's "Targets", which fittingly was about an aging horror star. It was released in theaters shortly before his death, on February 2, 1969.
Even though he is no longer with us, we have a treasure trove of classics to enjoy for the rest of eternity.  I havent even mentioned timeless classics such as Tower of London,  or The Lost Patrol.  His popularity today is carried in the hearts of his  faithfull fans, who's number deservedly continues to grow, ensuring the great actor's  legacy as one of filmdom's true masters.
Yes, you even begin to pity the monster, up and to the climactic ending of the movie.  On the heels of this was Son of Frankenstein, where the monster was written as lathargic and slow. Even so, Karloff was once again magnificant in his handling of the character, bringing to it a third and equally original persona.
Karloff's performance as the monster is both reviting and intense, adding a surreal darkness to the already dark and forboding tale.  Given that the monster was initially unable to speak, Karloff had to convey all of this communication and feelings thru grunts, growls, and expression. 

None could argue that his wasnt one of the greatest performances in the history of stage or screen. And it's rather ironic that his voice got him the boost he needed and yet his most significant role, one most lauded, is that of the mute monster. 

In my personal opinion, one of his greatest efforts was in The Black Cat, in which he costarred with Bela Lugosi. Possibly one of the eeriest movies you'll ever see, even by todays standards. Karloff and Bela did numberous films together, including The Raven and Black Friday.

I've always felt this teaming was underappreciated and not lauded as much as it should be. Nevertheless, Karloff and Lugosi excell in these masterpieces of terror.