It's been a while since I saw this asked!
I got this!
Basically, the Lumia line has a bunch of series.
400, 500, 600, 700, 800, 900, 1000, 1320, 1520, 2520 (the last three were the just one model)
There has been a little bit of variation and changes like how the 400 line was introduced relatively late, etc.
Basically, they indicate where on the Lumia smartphone line that the phone fits in - in regards to price/nice-ness.
The Lumia 900 was higher-end than the 800, which was higher-end than the 710, etc. This lets you quickly compare models mentally.
The second digit indicates generation with slight variations.
- x0x and x1x Lumias being WP7, WP7.5+ models. (2011)
- x2x Lumias were WP8 models (2012)
- x3x Lumias were WP8.1 models (2013)
- x4x Lumias were also 8.1 models but they were a year newer (2014)
- x5x Lumias were W10M/2015 models
And the last digit, the third one generally denotes the variant of the phone.
- The Lumia 925 was a slimmer, metal variant of the 920. The 928 was the Verizon model.
- The Lumia 625 was very different, but similarly positioned phone to the 620.
- The Lumia 730 and 735 are identical except the 735 supports LTE and Qi, while the 730 came in dual-SIM models.
- The Lumia 521, 525 and 526 are all slight variations of the 520. (one was for T-Mo/Metro-PCS and was slightly taller, the latter two had 1 GB of RAM...)
...
Now, there are some exceptions!
The Lumia 810 was a variant of the 820 and ran WP8 despite the name implying its an older model than it is.
The Lumia 1320 was a midrange/lower-end phablet. The Lumia 1520 was a high-end phablet, but it was on-par with the smaller 930 in terms of hardware.
The Lumia 532 and 435 are almost identical.
Instead of using a name in the 1500 series for the next high-end phablet, they went with a perhaps simpler 950 XL moniker.
The way they were positioned generally meant that the 900 series was the flagship line, the 800 series was an almost-as-good-but-cheaper lineup, the 500 was the bargain-basement-price deal, the 600 and 700 fit in between those...