I have come across claims that the continued fighting in Ukraine demonstrates that Putin wants to grab more land, or that he is actually fighting "deological war against imperial, decadent US" as Amb. Michael McFaul has put it.
I think Putin has no appetite for control of parts of Ukrainian land, which Russia would have to spend billions of dollars subsidizing, but rather he seeks to attain two objectives:
Objective Number 1: Rather, Russia's immediate minimum objective is to prevent Ukraine from military-political integration into West through keeping the threat of further fighting and loss of territory real until Kiev agrees to codify decentralization and non-bloc status in its constitution. (Those who think Russia would settle for a frozen conflict in Ukraine, should consider the following: there is nothing in NATO documents that explicitly bans NATO from admitting countries with unresolved territorial disputes. George Bush Jr almost got Georgia (and Ukraine) MAP to NATO at the 2008 NATO summit even though Georgia had 2 unresolved conflicts (S. Ossetia and Abkhazia). In a way, freezing of conflict, especially if sides are separated by peacekeepers, can even expedite Ukraine's integration into West – as it would remove immediate military threat and allow Kiev to focus on reviving economy and preparing its membership bid. In fact, that's what Alexander Motyl has advised Kiev to do).
Objective Number 2: The longer-term objective is to anchor neighboring post-Soviet states to Russia to facilitate its rise as an independent Eurasian pole in a multi-polar world, roughly on par with other poles, such as US, China, EU and maybe India further down the road. For that longer-term objective Russia doesn't want to reintegrate ex-Soviet reps into one unified state as this would require it to subsidize them, but rather wants them locked in a military-political alliance and EU-like economic union. While Putin realizes that animosities created by the conflict make it impossible to have Kiev acquiese to such post-Soviet integration anytime soon, he wants in the longer-run to at least ensure that Ukraine remains neutral and that Russia enjoys unhindered access to Ukrainian market and that it can continue to tap one resource – that Ukraine has and that is most valuable in Russia's eyes – millions of skilled Ukrainian workers: If you look at Ukrainian's GDP, you would realize it would increase the Russian-led Eurasian Union’s collective GDP by less than one-tenth. In comparison, adding Ukraine's 45.5 mn population to the Eurasian Union's 176 mn will increase it by more than a quarter.