We've defined cross training as any training that isn't running. With that in mind, Strength and Conditioning would come under the broad umbrella of Cross Training, however we want to split them up for a few reasons.
We want to think of cross training as an activity that gets the heart and lungs working, it makes you sweat and helps improve endurance.
Strength & Conditioning work might make you sweat, but more often that not it doesn't and is focused instead on complimenting your running by making you stronger, more mobile and conditioned. You won't always have a high heart rate when doing Strength and Conditioning work, and it might not give you that same buzz as a run does (which is probably why people ignore it!) but it is vital work to help make you a faster runner.
By splitting the two of them up, you know that a cross training session is focused more on getting the heart and lungs working, it is replacing a run in your week and will help improve endurance.
A Strength and/or Conditioning session will be a series of exercises that work on certain muscle groups, and will supplement your running.
Don't neglect either area of your training.