The Kingdom of Loathing is a turn-based game. Turns are called adventures. One adventure is long enough to go looking for something to do/kill in a location, to cook yourself some food, mix yourself a drink, or smith yourself something. Things that cost adventures are marked with the number of turns they take, like so: (1). Some actions will take more than one adventure, and are marked accordingly.
Other activities, such as purchasing a trophy, visiting a store, or talking to the Deep Fat Friars do not cost adventures, but they usually do not earn you items, stats or Meat without some sort of price - and if they do, it is typically limited to once a day. Gnollish signs can smith without using an adventure, but only if neither Armorcraftiness nor Super-Advanced Meatsmithing is required for the recipe.
Choice adventures that include a cancel option (such as this one) will usually not cost you an adventure to cancel.
Each day, at rollover, you will get 40 more adventures. A complete list of items which will modify that amount can be found at Extra Rollover Adventures. Suggestions for adventure-giving equipment based on your level of gameplay can be found at Maximizing Your Rollover Adventures.
You can get more adventures during the day by eating food, drinking booze, and using spleen items. Besides those common ways, some familiars will spontaneously grant adventures.
There is no limit to the number of adventures you can acquire. However, you are limited to 200 adventures after rollover is completed. Excess adventures are simply lost forever.
On your character sheet, you can see how many turns you've played in a given ascension, as well as how many adventures you've played since you created your account.