Seed
Seeds are the catalyst needed to grow any type of plant, without a seed you cannot grow anything. They come in a variety of shapes and colors, each one totally unique to the type of plant it will grow into. They are an important foundation to your garden, as with no plants, you will attract nothing but Whirlms and Sparrowmints looking to eat said Whirlms.
Seeds must be planted in the ground, but if on the wrong surface, the seed will refuse to be planted. With the exception of water flowers, all seeds can be grown in soil. So long as it is on a type of surface it can grow in, seeds will eventually plant themselves.
Digging a hole and planting a seed in it will cause the plant to grow twice as fast as it would otherwise, and is recommended if you want the resulting plant quickly.
Obtaining Seeds
You can obtain seeds from a variety of sources, but most often you will purchase them from Costolot's Store, or recieve them for free by talking to Seedos. Seedos is your primary source of new seeds as you progress through the game, as he will gift you new seeds before Lottie will stock them in her shop. Trouble in Paradise added the Seed Bag, a tool that allows you to purchase seeds at any time without needing to visit Lottie in her shop.
Tapping flowers with your shovel will make it drop both a flowerhead and the seed, allowing you to quickly duplicate flowers so long as you fertilize them. Weeds will also produce seeds upon being destroyed, often more than one. They usually drop multiple seeds from one plant, and along with being able to plant themselves quickly can easily cause weeds to grow out of control.
Uses
Seeds are most commonly used to grow plants. Seeds have other uses though, as seeds can be eaten by piñatas on their own. This can fulfill resident or romance requirements for some species, or even cause the piñata to change color or learn tricks. Seeds cannot be tinkered by Bart.