Clearing things up

• Conda is used to install packages (plotly is a package, numpy is a package, cufflinks is a package etc.)
• The list of available packages is found in some index, which in Conda parlance is called a channel. The default, "official" channel is maintained by Anaconda (Conda's developer), but anyone can open his own channel, and use it to distribute custom packages.

So, in the command you've shown: conda install -c https://conda.anaconda.org/plotly <package>

• The -c switch tells Conda to use a custom channel which happens to be called https://conda.anaconda.org/plotly¹
• <package> is the package to download from that channel.
• Specifying a channel is optional, and if you don't - then Conda will look in its default channels. But you must specify a package so that Conda knows what to install.

^{1 This is in fact a channel that belongs to a user called plotly, which is hosted on Anaconda Cloud, a free service offered by Anaconda to host custom channels.}

Back to your question

This channel seems to be unmaintained (the plotly package hosted there is very old). Given that, and the fact that the official plotly documentation says to use pip, that is what I would use.

Update: plotly updated their conda build, and added conda as an installation option in their GitHub repo (albeit not in their documentation website). So you can now safely use:

conda install -c https://conda.anaconda.org/plotly plotly

or even simpler (since Anaconda Cloud channels are searched automatically):

conda install -c plotly plotly

When using Anaconda Python, conda is the preferred way to install packages, but in any case both conda and pip should be run under Anaconda Prompt on Windows (Start --> Anaconda --> Anaconda Prompt). Installing packages from the standard command prompt when you have Anaconda is discouraged and can mess up your Anaconda installation.