Amazon's Alexa is the voice-activated, interactive AI bot, or personal assistant, that lets people speak with their Amazon Echo, Echo Dot and other Amazon smart home devices. Like Siri and Cortana, Alexa is designed to respond to a number of different commands and even converse with users. Alexa comes with more than a few capabilities: playing music, pulling up the weather or even reading news. But Alexa Skills are apps that give Alexa even more abilities, letting her speak to more devices even websites.
For tracking your food, you can use the Track by Nutritionix skill, which lets you record your food intake using your voice, or ask for caloric values of foods. (Alexa does the latter by default.) Say things like, "Alexa, tell Food Tracker to log a cup of almond milk" or "Alexa, ask Food Tracker how many calories are in two eggs and three slices of bacon."
The Capital One skill allows you to check your credit card balance or make a payment when one is due. This is secure: The skill performs security checks and requires you sign in using your username and password. Then, when you open the skill, you must provide a four-digit code to confirm your identity. Just be wary of who is around when using the skill -- anyone who overhears you say your personal key can access your banking or credit card info just by asking Alexa.
If you want to do some casual research for a future trip, you can get fare estimates using the Kayak skill. You can say, "Alexa, ask Kayak where I can go for $400" or "Alexa, ask Kayak how much it costs to fly from Los Angeles to Dublin." The skill will ask for additional information and eventually provide you with a series of options and price ranges.