In linguistics, a blend is a word formed from parts of two or more other words. The process is called blending and the result is a blend word.
A portmanteau word typically combines both sounds and meanings, as in smog, coined by blending smoke and fog. More generally, it may refer to any term or phrase that combines two or more meanings, for instance, the term "wurly" when describing hair that is both wavy and curly.
The word "portmanteau" was first used in this context by Lewis Carroll in the book Through the Looking-Glass (1871).
I'm not entirely sure if there is a technical distinction between portmanteaus and blends or if the latter is a hypernym of the former. There is some suggestion that portmanteaux specifically combine the beginning of the first word and the ending of the second. But I've been unable to confirm the validity of this distinction and I've found that they're mostly used interchangeably. Any information on this matter is welcome.
P.S. ELU has tags for both these terms: portmanteau-words, blend-words.