Many non-rhotics produce the r's when a word starting with a vowel follows. We discussed a bit the tense low front vowel, as in Sam and can , and the fact that some people contrast cot and caught , and some don't. Usually the air goes out , starting with your lungs. This is enough to communicate an intonation or a melody. We'll get back to that later.
Linguists divide sounds up into several large categories.
As with all distinctions, there are some fuzzy cases that blur the division, but let's ignore that for now. Vowels are formed with a relatively open mouth, with nothing blocking the passage of air through the main passage of the mouth. At the other end of the spectrum from vowels are obstruents , which are the hard core of the consonants. Nasals are produced with the velum down, permitting air to pass freely through the nose, while the mouth is closed at some point of articulation for a certain period of time. For example, think of some sounds that we use when speaking English which aren't on the chart.