Portuguese is a Romance language, closely related to Spanish. The theory goes that the differences between Spanish and Portuguese came on early, probably with the settlement of the two areas by different Roman legions. These legions were likely Romanized Italians who spoke Latin as a second language, their own languages being Italic languages, of which Latin itself was a branch. The original legions came from slightly different parts of Italy, and spoke their own languages when not dealing with the hoi polloi.
Portuguese grammar is similar to Spanish grammar. The exception is the future subjunctive tense, and only people who take life as easily as the Portuguese could figure out when to use such a thing. Almost all the vocabulary is the same, though the meanings are subtly different. Just as Chaucer used the word "freedom" to mean "generosity", Portuguese uses older meanings of words. Of course, there are differences. For instance, I found the word for chicken -- "frango" -- kind of amusing. A weird fact: the Portuguese understand Spanish perfectly where the Spanish can't understand Portuguese very well at all, kind of like a one-way mirror.
The main attraction of Portuguese is its phonetic system, which is positively bizarre. Anyone who knows Spanish can read Portuguese almost perfectly. But to hear it is quite another thing. It sounds like Russian, or Polish, or ... who knows what. Portuguese is trying to jump another branch, or perhaps to another tree.
Portuguese retains the "f" that Spanish converted into "h". The verb for "make" is "fazer", closer to the other Romance languages than the Spanish "hacer". However, what's an "n" in Spanish is usually an "m" in Portuguese, and the letter "b" is just as often a "v" (agradable = agradavel). What's really unique about Portuguese are all the contractions with definite articles, the syllables that get swallowed, and all the "shusshy"-sounding words. There's also the ubiquitously nasal "ão" sound. For instance, the word for "no" is "não", since the word "no" is already taken by the contraction of "em" + "o" (in + the). Any word that ends with "-tion" in English ends in "ção" in Portuguese, like "direção".
But the kicker, in my mind, is the bizarre lengths that the Portuguese will go to avoid the letter "L". I don't think I get it. The word for "white" in Spanish is "blanco". In Portuguese it's "branco". The word for "plaza" is "praça". A shift is normal, but it doesn't stop there. Where the Spanish say "salida" for "exit", the Portuguese say "saída", dropping not only the letter but also its place. My theory is that somewhere in the Middle Ages they used all the "L"s for kindling to get through a particularly bad winter. It just goes to show what happens when you mortgage everything for current gain -- the future becomes very unpresent.
In any case, having used Spanish for a long time it was extremely easy to read the signs, even though I couldn't understand a word that anyone was saying. People listened politely to my attempts at Portuguese, seemed to know what I meant, and were extremely kind when they didn't. If the only misunderstanding is a mere language, it's small enough for the Portuguese to take in stride. After all, you can still wave your hands to make your meaning clear.