Tower Defense (TD) games all have the same basic premise: defend a point by building structures to stave off the enemy waves. Once built, the structures target and attack enemies automatically. Each "tower" acts differently-- some attack a single target, some attack multiple targets, and some don't attack at all but provide support to others. The player decides which ones to build and where to build them.
Tower Defense Mechanics
Although the basic premise remains the same from game to game, the details of the games can vary greatly.
Much like the towers, the enemies also come in different types. They can vary in movement speed and health or can be resistant to certain types of damage. On occasion a boss may even appear with extremely high attributes, like General Disarray from South Park Let's Go Tower Defense Play!.
Paths and Nodes
In some games you can place your towers anywhere you want, like in Desktop Tower Defense or Trenched. Being able to place them wherever allows you to build paths-- to funnel troops the way you want. In other TD games, you'll have a set path and can only build towers in specific places. In PixelJunk Monsters, for example, you can only build towers where there are trees. And there are games like Vector TD that allow towers to be placed anywhere, as long as it's not on the path. So you can't change the path of the enemies, but the player isn't restricted to the predetermined nodes (or "squares" in the case of NinjaTown).
Tower defence games also allow you to upgrade your towers. Often times the towers go off a few different attributes such as attack damage, rate of fire, range and sometimes splash damage if it's something like a cannon. Sometimes upgrading a tower only increases one attribute, but in other games it could slightly increase all three, (or four). Some TD games require towers to be unlocked before they're available. Players typically need to spend the game's currency to make upgrades and/or unlock towers. Currency is usually earned by defeating enemies or completing a level.
Bonuses are things like increasing the amount of interest you get at the end of each round, or increase the attack power of all towers.
Interest is a percentage of money you get at the end of a round for still having cash. Sometimes you can purchase interest increases, so instead of getting ten percent on what you had at the end of the round, you can purchase an upgrade to fifteen percent.
You're of course defending something in a tower defense game. Whether it be lives, or villagers or a building. Sometimes you simply don't want the enemies to get past you. Either way, if enough do, if enough creep across the screen and make it to the exit, you'll lose.
Occasionally a tower defense game will have opponents who can damage you towers, which eventually can lead to their destruction. It's not too common but some TD games do have this mechanic; Plants vs. Zombies and Trenched are examples of this.
Common Tower Defense Modes
Some modes you'll see appear in many tower defense games. Modes that start you with a large sum of money. Or more challenging ones that only allow you to build so many towers.
Often times you'll see a mode that will start you off with a substantial amount of money, but you don't gain any more from enemies. It forces you to think strategically about which towers to build and where. How much you should upgrade each tower, and what bonuses you should buy, if any.
Some games include a mode where you can you have a tower limit. It's meant to provide challenge, and further strategic thought.