Tornado chart looks like a tornado... It is a pair of back-to-back bar charts, which could also serve as histograms.

In M&A investment banking you use them to show the combination of two companies in a merger. You can see how revenue down to FCF compare.

Some people use it for financial modelling as a graphical means of displaying the results of a deterministic analysis to show the most important input variables.

It is also known as an Age & Sex Pyramid or a Population Pyramid. You compare both sex and display the distribution of a population in all age groups and in both sexes. The X-axis is used to plot population numbers and the Y-axis lists all age groups.

Bullet Graphs are used to display performance data

The easiest way to think about them is for sales teams. You set what sucks to what is frickin awesome, then you can show the target and what was achieved.

They are basically a Bar Chart on acid, accompanied by extra visual elements to pack in more context.

They were developed by a dude called Stephen Few as an alternative to dashboard gauges and meters, because they didn't display enough information, were less space-efficient and were cluttered with "chartjunk".

The main data value is encoded by a length of the main bar in the middle of the chart, known as the Feature Measure. The line marker that runs perpendicular to the orientation of the graph is known as the Comparative Measure and is used as a target marker to compare against the Feature Measure value. So if the main bar has passed the position of Comparative Measure, you know you’ve hit your goal.

The segmented coloured bars behind the Feature Measure are used to display qualitative range scores. Each colour shade (the three shades of grey in the example above) are used to assign a performance range rating. So for example, poor, average and great. When using Bullet Graphs, it's ideal to keep the maximum number of ranges to five.

In M&A you would use this to show the contribution of sources of revenue over time. If you were looking at a asset manager you could see what comes from active vs passive assets.

They're one of the most common types of charts you will want to churn out

XY simply put, place something on two axis' so you can compare stuff. You show your paired numerical data and that one variable impacts the other. 

These are used for a huge number of reasons. Equity research analysts will have them in every broker report they write. Nerdy VCs blogging will discuss company valuations compared to others, economists will use them to show how relevant something is...

They are a collection of points placed using Cartesian Coordinates to display values based on two variables. The entire goal is to see if you can detect if a relationship or correlation between the two variables exists.

In fact, you can add a statistics thingamajig with r2 to see the correlation. If that number is close to one, your data set is highly correlated to the two variables. The strength of the correlation can be determined by how closely packed the points are to each other on along the line of fit (which you can fiddle with).

A waffle chart is a square display consisting of 100 smaller squares arranged in a 10-by-10 layout. It looks like a waffle... no shite. 

Each square is coloured according to the proportions, just like a pie chart. When the waffle is full it is 100%.