Not all treks are the same, and are hence classified based on these criteria: gradients during the trek, nature of terrain (like even paths, grass, gravel, stones, boulder in the trail, slipperyness of the trail), average altitude of the trek route, highest altitude of the trek, distance & duration of trek in a day, temperature during the trek, snow/ice on the trail, campsite locations, availability of water. Each of these criteria is evaluated and then a grading is achieved for a trek.
Trails are even and gently laid out. Usually runs through meadows and forest cover. Does not have big climbs or descents. Usually not more than 4-5 days long. Campsites have good access to water. Easy to exit from the trek at most times.
Trails are generally even but may have short steep climbs or descents. Days trek could last up to 6 hours. Trail may run into snow or climb above 14,000 feet (4,200 mts). Treks are usually 4-5 days long. Campsites usually have good access to water. Easy exits possible from some camps but not all.
Trails are generally even but could have rough sections. There could be long sections on snow, moraines or boulders. Steep climbs and descents sometimes over three hour long could be present on some days. One or two sections would require careful navigation. Trek could last 6-7 days. Temperatures at higher camps could dip in minus. Highest altitude gained could be above 15,000 feet (4,500 mts). Chances of altitude sickness possible. Easy exits possible from some camps but not all.
Trails could be uneven on some days. Trek days could be long stretching to 8-9 hours. Highest altitude gained could be above 16,000 feet (5000 mts). There could be some high altitude camps where air is thin and temperature in minus. There usually would be long walks and climb on snow. Glacier/crevasse crossing possible. Chances of altitude sickness high. Weather is generally unpredictable with rain/snow chances anytime. Easy exit options few.