SDKs are language-specific wrappers around APIs. They handle authentication, request signing, error handling, retry logic — so developers don't reinvent these per-API. Common: Razorpay JavaScript SDK, AWS Python boto3, Stripe Java SDK, Twilio Node SDK.
For Indian SMBs developing custom integrations, picking SaaS that has a SDK in your team's primary language is critical. Without an SDK, you write API integration from scratch — 5-20x more code, more bugs, slower.
SDK quality: well-maintained ones release updates monthly, have good docs + samples, handle edge cases. Poorly maintained SDKs are worse than no SDK (false sense of stability).