Syndicates are essentially in-game factions that you can align yourself with for various rewards. Who you choose will be aligned with certain other factions (and thus increase your standing with them) and opposed to others (decreasing standing with them). As such, you can’t possibly be in good standing with all Syndicates. Those you are in bad standing with will occasionally send a hit squad out to attack you in missions. This is more of an annoyance than anything else, although initially they will prove a challenge.
At first blush, Syndicates seem more trouble than they are worth – pledging to one just gives you a new list of daily missions that you can choose to do for standing, and increasing your level within a Syndicate costs credits. In fact, each level costs more and more – painfully so. But it’s important to take a look at what each Syndicate is offering, as that’s where the value lies…
Each Syndicate has a set of mods that are (mostly) exclusive to them. These are all specific to a weapon or warframe, and they generally provide huge bonuses or augments to warframe abilities. Without a doubt, there will be items at each Syndicate that you will want to chase, so my advice is to not make your decision lightly, and don’t make it based on the narrative “policy” of the Syndicate. Be selfish – read the offerings, and choose a Syndicate based on a target mod or weapon. Everyone’s playstyle is different, so I won’t mention what I chose – take your time and choose what feels like a good match for you. That said, most players will work towards either the three on the left of the Syndicate screen, or the three on the right, as these have some synergy and can all be levelled together – at the expense of the other three.