Conditions

The {% if %} tag evaluates a variable, and if that variable is "true" (i.e. exists, is not empty, and is not a false boolean value) the contents of the block are output:

{% if user_list %}
  Number of users in the list: {{ user_list|count }}
{% else %}
  No users found.
{% endif %}

In the above, if athlete_list is not empty, the number of athletes will be displayed by the {{ athlete_list|length }} variable.

As you can see, the if tag can take an option {% else %} clause that will be displayed if the test fails.

You can also inverse the check using not:

{% if not user_list %}
  No users found
{% endif %}

If you use just if and if not you can also apply filters on those values:

{% if user.name|strip %}
    username given
{% endif %}

Comparison

You can also check if two variables (or a variable and a string for example) match:

{% if item.id equals "index" %} class="active"{% endif %}

Or if they don't match:

{% if not page equals active_page %}
    ...
{% endif %}

In both cases you can't apply filters.