Glaire got the wounds correct. Of note, it is generally possible to get around 30 wounds for a space marine. Sometimes more if you take certain advanced specialties.
About the fragility of marines: Even they aren't tanks, they still keep to cover and engage enemies when its their benefit. Also, do not forget your special rules and abilities: solo mode abilities, fate points, etc. they all rack up the survivability and are options your basic human doesn't have access to.
Master, Elite, and Troop represent mainly the difficulty the enemy should represent to the players. Much like D&Ds creature level. Troops are cannon fodder generally, their weapons, options, and abilities shouldn't be a problem for the players outside of massive numbers. They are the rank and file and fill out basic everyday job positions like guards and patrols.
Elites are better trained they should pose more of a problem to characters. These guys use better tactics and should use more of the special actions in combat. They are still relatively easy enemies one on one, but just a few of these guys will pose a problem to the characters. These guys are liekly to be middle or middile-high level ranks or positions in your enemy forces. You can also expect elites from some forces to be operating with or leading troops, generally a large group of them. When this happens the elite tends to improve the troops performance.
Masters are equivalents to Bosses in video games. These guys have all the cool toys. They will usually be running the show. While they should be a threat to the players by themselves, it is far from uncommon for these guys to have a number of elites and/or troops with them, which makes encounters with them very difficult. Especially as their abilities makes all their forces even more powerful.
Example of troops: Platoon of Guardsmen led by a sergeant, a mob of ork boys, throng of cultists, pack of gaunts.
Example of Elites: Guard Lieutenant (more for his ability to lead troops than direct fighting ability), ork nob, chaos marine, warrior or genestealer.
You can expect the lieutenant and ork nob to be leading their troops and be a part of them though nobs may form their own elite only group. Chaos marines can lead cultists but may work alone or in a group as well. Tyranid warrios may work like chaos marines, genestealers are likely to work in their own groups, though it is not unheard of a few of them hiding out in a gaunt unit.
Examples of Masters: Lord General (if he shows up, he'll have an army with him), ork Boss, Chaos marine captain or major daemon, Hive Tyrant.
Any of these may be leading their own forces though the chaos marine/daemon is more likely than others to work alone. Hive Tyrants should almost always have a bodyguard of some kind and a Lord General should almost never be used (assassinating him in his hotel etc for instance) unless he has a full military force with him.