Los Nevados is a national park centered around some snow covered volcanic peaks; Nevado Del Ruiz, Nevado del Tolima and more. The park is run by a "concession" - Concesion Nevados - Which actually is the travel agency Aviatur. Although not ruining the park entirely, it does feel more like an alpine resort than a national park. In practice this means that the park is privatised and that the goal of the park is to shuttle as many people as possible up the roads and down again.
Reasonably cold. Expect frosty mornings (0 - -10 C) year around. In the wet season, expect clouds continuously, in the "dry season" expect clouds and rain 10 AM to 3 PM, and clearer conditions before and after.
The usual way to get in is from Manizales. From there you have several options: Go on a tour and jump off (120'000 COP incl. entrance and breakfast), Taxi (300'000 COP) take a bus to the intersection and hike/hitchhike up. The last option is the best one if you have the time, as you would gradually acclimatise to the altitude, not pay excessively, and avoid the extremely commercial feel of the Concesion.
The only outdoors store in Manizales - Amazonas Deportes near the cathedral has a pathetic selection of gear another option is hire some specialized guides, these guides offer gear rental service, this little business is called "Zion Integral Mountain Guides" and you can find them in the facebook.
There are campsites at El Cisne and the Chalet. You are required to have a tent if sleeping at the campsites, if you sleep in a bivvy bag they make you sign a waiver that if you die it's not their fault, and that they warned you. I would recommend getting a tent anyway because it rains so much that you will need it to keep things dry.
Temperatures at El Cisne campsite are said to drop down as far as -17°C at night so ensure you pack warm clothes and have a good quality tent.
Maps are not available anywhere in Manizales or the park itself. The only map you will get is the cartoon map you see on the website of the concession. Apparently the only place you can buy a map is in Bogota, from the Geogaphic institute itself (Sigac).