This is because the common name "Geranium" is often used about two different groups of plants. This article is about growing the annual type from seeds.
If you are still not sure of the difference, allow me to summarily detour into the area of taxonomy, which is the branch of biology which names and groups organisms according to their characteristics and evolutionary history.
Biological Nomenclature
In the mid 1700 a system was devised, which is still in use today, which catalogues all living organisms into a hierarchy of classes from the broadest "kingdom" (such as the kingdom of bacteria, kingdom of animals, kingdom of plants, etc) to more specific classes such as "order", "family", "genus", and "species". The further down in the classes you go, the more in common the plants will have.
Both types of Geranium belong to the same hierarchical class called "family": Geraniaceae, which covers 6 different genera of plants. The two largest genera in this "family" are "Geranium" and "Pelargonium".
To illustrate in terms which may be easier to understand: humans belong to the "animal" kingdom. In this kingdom, we find the order of "primates" which again consist of various families. In the family "hominoidae", we find several genera.
We belong to the genus (plur: genera) "Homo" (humans are a species in this genus). Other genera in the "hominoidae" family, are "Pan" and "Pongo" (great apes).
However, as some of these latin names can be quite difficult to remember, we often use common names instead, such as human beings instead of homo sapiens and great apes instead of pan or pongo.
Common names are also used with plants. The common name "geranium" is used as a description about plants of both the genus 'Geranium' and of plants belonging to the genus 'Pelargonium' (in the same manner we commonly refer to both Pan and Pongo as great apes).
Plants of the genus "Geranium" are hardy outdoor plants, usually called "hardy geraniums" or cranesbills. They are typically mounted plants which many small flowers covering the plant. You may also have heard them referred to as "true geraniums". This is not because they are better or superior to pelargonium, but simply because the common name "geranium" is the same as the genus "Geranium".
The other genus "Pelargonium" covers several individual species, but includes the very familiar garden geranium (zonal geranium). Think of plants with scalloped foliage with dark zonal markings and with pom-pom like flowers held above the foliage by flower stalks (see pictures). Another popular type of pelargonium is Martha Washington, or Regal, Pelargonium which are typically grown as a house plant.
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