They're both polyunsaturated fats. While an over-simplification Omega3s (which ALA is) tend to be anti-inflammatory and have tons of positive effects of cardiovascular health (up to a point - more on this later) while Omega6s, which are necessary for our survival, do tend to have an inflammatory effect.
The goal is to be high-ish in Omega3s without getting mercury poisoning and low in Omega6s.
The modern diet - so if you go out to eat at all, if you cook with corn/canola/soybean/cottonseed oils or shortenings like Criso, if you use 90% of most modern pre-made marinades, salad dressings, cereals, chips, crackers, cookies, and even most bread (not including some brands of butter bread), they all load up on "seed oils" (Meaning LA).
When people say a good diet is rich in whole foods and low in processed foods - this is what is at the heart of it. Processed foods tend to be high in omega6s, high in sugar, and high in sodium,. The latter 2 are largely for taste, or in salt's case taste and preservation.
Omega6s are used b/c the oils are super cheap. These are oils that couldn't exist without modern industry, in the bulk in which we produce them. (unlike say..beef tallow, coconut oil, butter, olive oil, ghee, duck fat, etc.) And initially they were sold by non-food industries as a way to maximize profits on byproducts from other industrial processes.
They have incredibly high smoke points and they don't go rancid or spoil, so if you want to make say... Cheeze-Its or Pringles, it's a perfect choice for production - it's cheap, it's stable, it doesn't burn easily, it doesn't spoil, etc..
What's worse is we've radically changed what we feed animals too - chickens, pigs, cows, and now all of the meat we eat also has a much higher omega6 profile than it ever did before these seedoils came along or "corn" became the staple of our lives from ethanol to chicken feed.
Foods high in Omega3s tend to be fatty fish, the low mercury types would be salmon and sardines, mostly. Tuna is good too but higher in mercury. and you'll want to eat the wild versions of these b/c at least in Salmon's case it also contains an antioxidant called Astaxanthin, which is an amazing anti-oxidant. Salmon get this from eating Krill (Krill get it from eating algae - and it's why krill and salmon are red/pink).
Unlike most anti-oxidants, Astaxanthin doesn't flip to being pro-oxidant under any circumstances and it, particularly, prevents Poly-Unsaturated Fats from Oxidizing. Which is really important since they oxidize so easily ( this is why Omega3 based oils, for example, aren't good cooking oils).
Fish are high in EPA and DHA, which are forms of Omega3. ALA is the type found in flax or papaya or some other sources. I'd mention chia seeds but being as they're higher in oxylates than spinach, i reject them as a health food. ALA is a poor Omega3. Your body can't use it directly and it must be converted into EPA and DHA and it's done so at a really poor conversion rate. Something like only 5-10% of the Omega3s in ALA form ever get converted to a form that your body can use.
And while there's 1000s of blogs and videos going on about the benefits of Omega3s, which there are many, let me also give warning that high doses of EPA (pure EPA at 4-7g) given to patients in a clinical trial, for years, gave them heart arrhythmias. While no fun, the main cause of death from heart arrhythmias is stroke, and funny enough, Omega3s lessen the risk of death from stroke. So you'd get a heart arrhythmias but still be less likely to die from it. The lesson here, though, is the poison is in the dose.
The more you read Fire in a Bottle (blog linked in first comment), you'll notice that the human body really isn't meant to process mass amounts of PUFAs at all.
This is a fantastic talk on Omega3s:
https://www.youtube.com/watch?v=-f-CFQxaUY4
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