What is a side-quest? Well, that’s a trickier question that it sounds… a side quest in Skyrim is a quest that is big enough that it’s not classed as a “Miscellaneous Objective,” a quest that isn’t a questline quest (i.e. main quest, guild quest and the civil war quests) or one of the many Daedric quests. It’s tougher than it sounds in all honesty and tricky to work out. Out of 86 quests (at last check), only 12 of those were side missions. As long as it doesn’t fall under the three points above though and is fairly substantial (they’re not short missions!), then it should register as a side-mission.
Things like the quest for the trader in Riverwood that sends you to Bleak Falls Barrow to retrieve the golden claw are side missions, visiting somewhere because someone told you to are not.