Strength = Nicotine
Both Tobacconist University and Holt's Cigar 101 equate cigar strength with nicotine content. Holt's "The strength of a cigar exclusively refers to its nicotine content. Because premium cigars are handmade entirely from whole-leaf tobaccos, their nicotine content is not measured in a laboratory setting like cigarettes...."
The TU explanation has a bit more depth: "Nicotine is a naturally occurring organic compound in the same family of substances (alkaloids) as caffeine: it is found in tomatoes, green peppers, potatoes, and tobacco. Traditionally, we do not think of nicotine as something we touch, but it is definitely something we feel. As with caffeine and alcohol, every person has a different tolerance level to nicotine: too much nicotine can induce nausea and a light head: if this every happens to you, consume a little sugar and it will subside. The effects and/or level of nicotine will determine the strength of a cigar."
The mild cigar would be Napoleon Dynamite, says Gwaltney. “Though he talks a big game about his bow staff skills and killing wolverines, deep down he's just a nice guy with smooth dance moves.”
The medium? James Bond. “He’s not a super-strong guy but does an excellent job of using his head, along with a great collection of gadgets, to get the job done,” Gwaltney says.
Bold? “Thanos is the embodiment of a full-bodied cigar. He is primarily concerned with only strength and destruction and has zero time for the weak.”
Leaves that get more direct sunlight tend to have more nicotine. The included picture is from La Aurora Cigars. They are the original cigar factory in the Dominican Republic and actually have a training kit! While there are really two primary kinds of tobacco plants (each with different names for the sets of leaves), we at Bailey's discuss Ligero, Veso, and Seco leaves - with ligero being the strongest.
Body, strength, and flavor are all relative. We'll get to flavor soon enough. The best way to determine what you like is to smoke a variety of cigars. Each cigar smoker's journey is personal. Enjoy your's!