Short answers, pulled from the story.
A portmanteau word is a word formed by combining the sounds and meanings of two or more existing words, such as smog from smoke and fog. The term was introduced by Lewis Carroll in Through the Looking-Glass in 1871, where Humpty Dumpty compares the double meanings packed into such words to a two-part suitcase that opens into equal halves.
A compound word preserves both source words in full, as starfish preserves both star and fish intact. A portmanteau or blend contains at least one splinter, a fragment of a word rather than the whole stem, as brunch retains only parts of breakfast and lunch.
Well-known English portmanteaus include brunch (breakfast and lunch, first printed in Punch in 1896), motel (motor and hotel), smog (smoke and fog), Tanzania (the 1964 name formed from Tanganyika and Zanzibar), Brexit (Britain and exit), and Microsoft (microcomputer and software).
Lewis Carroll coined chortle in "Jabberwocky" by blending chuckle and snort. Carroll himself introduced it as one of the original portmanteau words when explaining his invented vocabulary to readers.
Many languages form portmanteaus actively. Modern Hebrew blends tala and völva into tölva for computer in Icelandic, while Israeli Hebrew uses ramzor (traffic light) from rémez and or. Indonesian compresses five city names into Jabodetabek for the Greater Jakarta area. Japanese commonly retains two morae from each source word, and Spanish company names such as Pemex and Cemex follow the same pattern.
Portmanteau is the English term, drawn from a French word for luggage, introduced by Lewis Carroll. French linguists felt the term had drifted too far from Carroll's original intent once exported into English, so France uses mot-valise, meaning word-suitcase, to name the same phenomenon.