DescribeSObjectResult: Check whether the object is Custom or Standard

List<String> allCustomObjsList = new List<string>();
List<String> allStandardObjsList = new List<string>();

for(Schema.SobjectType obj : Schema.getGlobalDescribe().values()) {
    Schema.DescribeSobjectResult objResult = obj.getDescribe();
    if(objResult.isCustom()) {
        String strObjName =objResult.getname();
        allCustomObjsList.add(strObjName);
    }
    else {
        String strObjName =objResult.getname();
        allStandardObjsList.add(strObjName);
    }
}

System.debug('Custom Objects List: ' +allCustomObjsList);
System.debug('Standard Objects List: ' +allStandardObjsList);

If you need to check for a specific object, execute the below code:

Schema.DescribeSObjectResult describeAccount = Schema.sObjectType.Account;
System.debug(describeAccount.isCustom());

Similarly you can use other methods for describing sObjects, the list of which can be found below:
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_methods_system_sobject_describe.htm