Salesforce provides REST API which can be used to integrate Salesforce with any external system. It all depends what is your existing language. Salesforce also provides SDK for different languages such as PHP, JAVA, .NET
You can check Example section of REST API to understand how easy it is to create, update, delete records in Salesforce using REST API.
If you want to send data out of Salesforce you can also do httprequest callout which can post data to your external webservice once any record created/update inside Salesforce.
You can also use external ID concept of Salesforce which can work as primary key between your external system and Salesforce.
Using the Lightning connector you can show external system data on real time basis inside Salesforce.
You can also use Streaming API for your requirement.
Salesforce work in multitenant architecture it has some limit so you should opt your option wisely.
here are the resource for API limits