There are different levels to which borrowed words become assimilated into English. Some words like ensemble or bratwurst are obvious borrowings and partially retain their foreign spelling and/or pronunciation. At the other extreme are words like marmalade  or mayor that have become Anglicized to the extent that we can't tell from the words themselves (pronunciation or spelling) where they are borrowed from, or even that they have been borrowed. The level of assimilation depends on time and usage - the longer since it was borrowed and the more it is used, the greater its degree of "Englishness" and the more familiar it appears to us. For example, words from Greek like area and problem are more familiar than euphoria and persona; and from French marriage has been Anglicized while montage retains its French pronunciation.
Many borrowed words result from the age of exploration in which new things were discovered from around the world and named from words taken from the local language. Chimpanzee is from the West African language Tshiluba, geyser from Icelandic, sauna from Finnish, and futon from Japanese. Closer to home, Saskatoon berries and pemmican are from Cree words, as is the name of my province Saskatchewan .
Similarly words may be borrowed because there is no English equivalent, even though the object or notion is well known. My favorite example of this is the German word Ohrwurm (literally "ear-worm") for that tune you just can't get out of your head. Other loanwords may already have an English word for it, but the new word is more descriptive (entrepreneur) or adds a particular shade of meaning (scarlet and vermillion from French).
The terms "borrowing" and "loanword" seem rather odd in reference to words, as the loaning languages don't have to give up their words and there is no expectations of having to pay them back (sort of like your teenager "borrowing" $20 to go to the movies). English however has returned the favor many times and loaned words to languages from which it has previously borrowed. French now has le weekend and cool, Japanese intanetto and wado purosessa , and German has die bluejeans and der blogger.
 from Greek
 from Portuguese
 A story goes that some American hunters pull into a gas station and ask the attendant where they are. The attendant replies "Saskatoon, Saskatchewan". One hunter then turns to the driver and says "I told you we went too far north - the natives don't even speak English".
 Try saying them out loud without the final "o".