Try the Richters web site where they do have photos of these herb plants.

Yarrow is "Achillea" Tansy is "Tanacetum vulgare"

Yarrow is Achillea millefolium, Family: Asteraceae/Compositae.

Tansy is Tanacetum vulgare, Family: Asteraceae/Compositae.

(There is currently a move to re-classify plants, hence some changes that you'll find in the botanical names, in particular the Family names. It need not bother you unduly. In this case, Compositae is the 'old' name, Asteraceae is the 'new' name.)

You will find in each botanical family, a huge range of plants. Within the family, you have a Genus, in these cases 'Achillea' and 'Tanacetum'. The Species name is tacked on to the Genus name, and describes a particular feature of the plant, thus identifying it precisely.

So - you get lots of plants with the Genus name, and lots of plants with a Species name, but only one plant with the same Genus+species name. For example, you get Tanacetum balsamita (costmary) and Tanacetum parthenium (Feverfew) - obviously members of both the same Family, and of the same Genus, but differentiated by their Species names.

Think of it a bit like this: You have the Smith family. You can have 2 members of that family called John. But one is 'John with blonde hair' and one is 'John with curly hair'. Clearly 2 different people, closely related, each resembling each other but with a strong distinguishing feature. Get the idea?

'Millefolium' means 'thousand-leaved'.

See this site:
http://www.all-creatures.org/picb/wfshl-yarrow.html

'Vulgare' means 'common'.

As you see, you can get several Species of plants in the same genus, each with its own common name. Confusingly, you can get different plants having the same common names - which is why it's important to use the botanical name wherever possible - especially if you want to eat it, or use it medicinally.

So, while your plants are closely related, they are different from each other.

As for the plant labelled simply 'Achillea', it's impossible to know precisely without being given a Species name - but it's almost certainly a yarrow (perhaps a cultivar with a different flower colour) or very near relative.

Incorrect or inadequate labelling of plants is one of my bug-bears!! I will not buy a plant that doesn't tell me EXACTLY what it is I'm buying!

Here's a link to a site listing some botanical names and their meanings. It's only one of thousands of similar sites.

Here is a link that might be useful: botanical names and their meanings