While very similar, Alerts and Subscriptions fulfill two very different needs that will be discussed here.
A Subscription sends an image copy of the subscribed report at a specified interval to the users subscribed. Subscribed users must be valid, licensed users in Insight. The intent is to provide regular updates of reports that are pertinent to the subscribed users taking away the need to visit Insight on a regular basis and adjust filters and parameters. As image snapshots of the data, the intent is to provide quick indication of significant changes snapshot to snapshot. It is not intended to provide a full report complete with underlying raw data.
Subscription notifications do not attach the original data in an Excel or CSV format. This is primarily due to customer email attachment policies that Insight may inadvertently violate. It may also include restrictions on attachment file types based on who the sender is. There is also the possibility of specifying a data set which results in a very large (70+ MB) attachment, which typically makes attachment policies fussy.
Subscriptions do not produce Excel or CSV exports of the Report(s) that subscribed to
Alerts differ in that instead of a timed schedule for notification, the notification occurs when a condition is met. For example, if you are concerned with the average minute utilization of a particular employee's extension since you suspect service abuse. You would create an alert specifying the field of interest and a threshold value and condition. Once the condition and threshold are satisfied, you are notified immediately of the occurrence.
When creating a Subscription be sure you are doing so on a report that uses Relative Dates then make sure that the Relative Date range set is based off today. This ensures that when the Subscription does notify you, it's doing so always looking backward relatively.
Creating a Subscription on a report with Absolute Date Ranges will never update and result in the same summary snapshot being sent to you every time.