Although this is covered in great detail in many places, including the comprehensive fact sheet, here is just a quick recap:
When you select Info on a wormhole, you will see a description that the WH leads to Unknown Space (Class 1-3), Dangerous Unknown Space (Class 4-5), or Deadly Unknown Space (the dread Class 6). As a basic rule of thumb, here are some basic facts on the WH classes:
Class 1 and 2: Definitely solo-capable. No warp-disruption by Sleepers. Plex sites are almost always "Perimeter" sites, which are the easiest - no more than one Sleeper BS in any individual spawn (NOTE: Be sure to research what the "trigger" sleeper is to make sure you clear as many sleepers as possible, especially the Sleeper BS, before the next spawn). Less Radar and Mag sites to harvest. Still bountiful in Ladar and Grav sites, and rich in ore types, but not in volume. For example, I have seen Spod and Crokite, but in small volumes in Class 2's. IMHO, forget Class 1. Use a Class 2 to prepare for Class 3.
Class 3: I would say they are solo-capable if you are working with your alt character as a shield/armor repairer role. Even with the best fits, many sites can spawn sufficient DPS to force you into warp-out. However, the Emergent (frigate) and Awakened (Battlecruiser) units will commonly web and warp-disrupt, so unlike Class 1 and 2, be sure to bring ECM or take down warp-scramble capable ships first. Plex sites are typically of the "Frontier" variety, but you will still see "Perimeter" and you will begin to run into the much more difficult "Core" sites, which are definitely NOT solo-capable. I think the increase in loot value compensates for the risk, but I would "cut your teeth" on a Class 2 first to really get versed in ship types, and most importantly, become well practiced in managing shield/armor repair while keeping a constant scan going for gankers.
Class 4+: Frankly, I have only visited and been a rail-bird, watching epic 10+ member squads battling it out with sleeper sites. Truly the land of organized squads with the proper mix of DPS ships and Logistics/Reppers. Pretty much all "Core" plex sites, with the occasional "Frontier".