Aggressive, Introduced, Invasive—What do they mean? 
All invasive plants are introduced, but not all introduced plants are invasive. Aggressive? It's that plant that you may want, but you don't want taking over your garden, it grows too vigorously to be planted just anywhere. Some of the most irritating and problematic invasive species are also aggressive, such as creeping bellflower (Campanula rapunculoides).

According to the invasive species page on Alberta.ca: Invasive species are non-native species that have been introduced, that threaten our ecosystems and biodiversity.

It's the second part of the invasive species definition that is critical. We have many introduced (non-native) species of plants in our gardens and local environment that do not hinder or threaten the natural biodiversity of the ecosystem. In fact, many of them are useful to us and the native fauna. They are also plants that native plants can co-exist with and enhance biodiversity. As people and animals move, plants come along for the ride. Some are deliberately brought from one area to another as a source of food or medicine, others attach themselves to fur or garments and hitch a ride. If anyone has brushed against a forget-me-not (Myosotis asiatica) seed pod, you certainly know how clingy a seed pod can be. Any non-native plant (one that has not evolved and originated here) is an introduced plant.

Many of our food and ornamental garden plants have their origins outside Alberta but can be grown here without concern. However, as gardeners, we need to pay attention to which plants self-seed or spread by other methods (including being carried by wildlife) and propagate outside of the intended growing area. These plants have the potential to become a problem.
 
Aggressive plants have good adaptions for spreading by seed, rhizome, or root. A native plant that can be an aggressive plant in the garden is common yarrow (Achillea millefolium). Given the opportunity, it will grow almost anywhere, and its root is difficult to remove once the plant is established. It is important to think of the "right plant, right place" mantra when planting aggressive plants. They can be controlled by planting them in a difficult-to-grow area of the garden, or in a container, and by removing spent flowers. But if aggressive plants are planted in an area with their optimum growing conditions, you will quickly find they are displacing all other desired plants.

If you want to know more about plants designated as invasive in Alberta, visit the Alberta Invasive Species Council website (abinvasives.ca). Note: it is important to look at local invasive species information as designations vary among provinces and territories.