I'm looking for a down jacket. They all seem to come with numbers but I'm not sure what they mean. For example :
- 90/10 Down
- Fill Power 700
What do they mean?
They are basically measures for the quality of the down fill.
The 90/10 part refers to the mixture of down and feathers. As down contains nearly no rigid structure, one adds some amount of feathers to give the whole filling some more stability. The example of 90/10 means 90% down, 10% feathers and seems to be quite a typical mixture. I'm not sure about the exact numbers, but mixtures worse than 80/20 or so would give significantly worse insulation.
The fill power, often with the unit cuin or cubic inch per ounce, defines, how good the downs regain their loft after compression. To test this, an ounce of the respective down is compressed in a cylinder for 24 hours. After release of the pressure the volume that the downs regained is measured in cubic inches. Good down for outdoor clothing typically has a fill power of at least 650 to 700 cuin, very good (but also very expensive) products reach fill powers up to 900.
Your question is also addressed at http://www.outdoorgearlab.com/Down-Jacket-Reviews/buying-advice
High-end jackets often use lighter-weight fabrics and high-quality down. The only practical differences between them may be in features (hood? two-way zipper? water bottle pocket?) and exact fill weights. Lower-end models may use heavier, more durable fabrics and lower quality down.
The most effective means of objectively testing the jackets will require sending them to a lab for testing on a thermal manikin. Since you probably don't have that available, there are a few numbers you should look for.
A final check is whether the jacket is "sewn-through" or "box-baffled"; the former allows for cold spots as there is no insulation at the seams. For a lightweight "sweater" this is of minimal impact, but if you're looking for something to use in serious cold you want baffled construction. Also keep in mind the temperature to which a jacket will be suited is very dependent on activity. Sitting around talking is quite different from snowshoeing, or even doing basic camp chores.
 E.g. duck instead of goose down, or lower fill power down.
 For fill weights I use the following rough examples, based on 800 fill power down: 75-125g: light down "sweater" 200-250g: mid-weight winter jacket 350g+: expedition-weight parka (Jackets using lower fill power down will likely need additional weight to provide the same amount of insulation)